add some parametrized tests for iter_lines()

Write a list of different chunk splits and their expected results
to test against, using ianepperson's breakdown as specification:
https://github.com/kennethreitz/requests/pull/2431#issuecomment-72333964

tests/test_requests.py

@@ -1318,10 +1318,7 @@ class TestRequests:
         ]
         mock_data = ''.join(mock_chunks)
 
-        def mock_iter_content(*args, **kwargs):
-            '''Fake difficult data.'''
-            for chunk in mock_chunks:
-                yield chunk
+        mock_iter_content = lambda *args, **kwargs: (e for e in mock_chunks)
 
         r = requests.Response()
         r._content_consumed = True
@@ -1332,13 +1329,45 @@ class TestRequests:
         # Because '\n' is a single line-end, when `iter_lines()` receives
         # the chunks containing a single '\n', it emits '' as a line -- whereas
         # `.splitlines()` combines with the '\r' and splits on `\r\n`.
-        assert list(r.iter_lines()) != mock_data.splitlines()
-
+        result = list(r.iter_lines())
+        assert result != mock_data.splitlines()
+        assert result[2] == ''
+        assert result[4] == ''
         # If we change all the line breaks to `\r`, we should be okay.
         mock_chunks = [chunk.replace('\n', '\r') for chunk in mock_chunks]
         mock_data = ''.join(mock_chunks)
         assert list(r.iter_lines()) == mock_data.splitlines()
 
+
+    @pytest.mark.parametrize(
+        'content, expected_no_delimiter, expected_delimiter', (
+            ([''], [], []),
+            (['line\n'], ['line'], ['line\n']),
+            (['line', '\n'], ['line'], ['line\n']),
+            (['line\r\n'], ['line'], ['line', '']),
+            (['line', '\r\n'], ['line'], ['line', '']),
+            (['a\r', '\nb\r'], ['a', '', 'b'], ['a', 'b\r']),
+            (['a\n', '\nb'], ['a', '', 'b'], ['a\n\nb']),
+            (['a\r\n','\rb\n'], ['a', '', 'b'], ['a', '\rb\n']),
+            (['a\nb', 'c'], ['a', 'bc'], ['a\nbc']),
+            (['a\n', '\rb', '\r\nc'], ['a', '', 'b', 'c'], ['a\n\rb', 'c'])
+        ))
+    def test_response_lines_parametrized(self, content, expected_no_delimiter, expected_delimiter):
+        """
+        Test a lot of potential chunk splits to ensure consistency of
+        iter_lines(delimiter=x), as well as the legacy behavior of
+        iter_lines() without delimiter
+        https://github.com/kennethreitz/requests/pull/2431#issuecomment-72333964
+        """
+        mock_chunks = content
+        mock_iter_content = lambda *args, **kwargs: (e for e in mock_chunks)
+
+        r = requests.Response()
+        r._content_consumed = True
+        r.iter_content = mock_iter_content
+        assert list(r.iter_lines()) == expected_no_delimiter
+        assert list(r.iter_lines(delimiter='\r\n')) == expected_delimiter
+
     def test_prepared_request_is_pickleable(self, httpbin):
         p = requests.Request('GET', httpbin('get')).prepare()