[core] Refactoring IOFormats so there is one and only obvious way to send it.

This is the commit where I admit that having more than one input/output
format for readers and writers was complicating the code too much for a
very small gain, and that it would be easier to only have one way to do
it.

So such way is now:

- Returning (or yielding) a dict if you have key-value type collections.
- Returning (or yielding) a tuple if you have a list-type collection.
- Returning (or yielding) something else otherwise, which will continue
  to work like the old "arg0" format.

IOFORMAT options has been removed in favour of a RemovedOption, which
will complain if you're still trying to set it to anything else than the
one value allowed.

tests/io/test_csv.py

@@ -3,25 +3,19 @@ import pytest
 from bonobo import Bag, CsvReader, CsvWriter, settings
 from bonobo.constants import BEGIN, END
 from bonobo.execution.node import NodeExecutionContext
-from bonobo.util.testing import CapturingNodeExecutionContext, FilesystemTester
+from bonobo.util.testing import FilesystemTester, BufferingNodeExecutionContext
 
 csv_tester = FilesystemTester('csv')
 csv_tester.input_data = 'a,b,c\na foo,b foo,c foo\na bar,b bar,c bar'
 
 
-def test_write_csv_to_file_arg0(tmpdir):
+def test_write_csv_ioformat_arg0(tmpdir):
     fs, filename, services = csv_tester.get_services_for_writer(tmpdir)
+    with pytest.raises(ValueError):
+        CsvWriter(path=filename, ioformat=settings.IOFORMAT_ARG0)
 
-    with NodeExecutionContext(CsvWriter(path=filename, ioformat=settings.IOFORMAT_ARG0), services=services) as context:
-        context.write(BEGIN, Bag({'foo': 'bar'}), Bag({'foo': 'baz', 'ignore': 'this'}), END)
-        context.step()
-        context.step()
-
-    with fs.open(filename) as fp:
-        assert fp.read() == 'foo\nbar\nbaz\n'
-
-    with pytest.raises(AttributeError):
-        getattr(context, 'file')
+    with pytest.raises(ValueError):
+        CsvReader(path=filename, delimiter=',', ioformat=settings.IOFORMAT_ARG0),
 
 
 @pytest.mark.parametrize('add_kwargs', ({}, {
@@ -30,7 +24,7 @@ def test_write_csv_to_file_arg0(tmpdir):
 def test_write_csv_to_file_kwargs(tmpdir, add_kwargs):
     fs, filename, services = csv_tester.get_services_for_writer(tmpdir)
 
-    with NodeExecutionContext(CsvWriter(path=filename, **add_kwargs), services=services) as context:
+    with NodeExecutionContext(CsvWriter(filename, **add_kwargs), services=services) as context:
         context.write(BEGIN, Bag(**{'foo': 'bar'}), Bag(**{'foo': 'baz', 'ignore': 'this'}), END)
         context.step()
         context.step()
@@ -42,61 +36,24 @@ def test_write_csv_to_file_kwargs(tmpdir, add_kwargs):
         getattr(context, 'file')
 
 
-def test_read_csv_from_file_arg0(tmpdir):
-    fs, filename, services = csv_tester.get_services_for_reader(tmpdir)
-
-    with CapturingNodeExecutionContext(
-        CsvReader(path=filename, delimiter=',', ioformat=settings.IOFORMAT_ARG0),
-        services=services,
-    ) as context:
-        context.write(BEGIN, Bag(), END)
-        context.step()
-
-    assert len(context.send.mock_calls) == 2
-
-    args0, kwargs0 = context.send.call_args_list[0]
-    assert len(args0) == 1 and not len(kwargs0)
-    args1, kwargs1 = context.send.call_args_list[1]
-    assert len(args1) == 1 and not len(kwargs1)
-
-    assert args0[0].args[0] == {
-        'a': 'a foo',
-        'b': 'b foo',
-        'c': 'c foo',
-    }
-    assert args1[0].args[0] == {
-        'a': 'a bar',
-        'b': 'b bar',
-        'c': 'c bar',
-    }
-
-
 def test_read_csv_from_file_kwargs(tmpdir):
     fs, filename, services = csv_tester.get_services_for_reader(tmpdir)
 
-    with CapturingNodeExecutionContext(
+    with BufferingNodeExecutionContext(
         CsvReader(path=filename, delimiter=','),
         services=services,
     ) as context:
         context.write(BEGIN, Bag(), END)
         context.step()
+        output = context.get_buffer()
 
-    assert len(context.send.mock_calls) == 2
-
-    args0, kwargs0 = context.send.call_args_list[0]
-    assert len(args0) == 1 and not len(kwargs0)
-    args1, kwargs1 = context.send.call_args_list[1]
-    assert len(args1) == 1 and not len(kwargs1)
-
-    _args, _kwargs = args0[0].get()
-    assert not len(_args) and _kwargs == {
+    assert len(output) == 2
+    assert output[0] == {
         'a': 'a foo',
         'b': 'b foo',
         'c': 'c foo',
     }
-
-    _args, _kwargs = args1[0].get()
-    assert not len(_args) and _kwargs == {
+    assert output[1] == {
         'a': 'a bar',
         'b': 'b bar',
         'c': 'c bar',