Fix the round trip conversion issue

Code cell was transformed to a markdown cell in py format
#339

jupytext/cell_reader.py

@@ -38,7 +38,9 @@ def paragraph_is_fully_commented(lines, comment, main_language):
     """Is the paragraph fully commented?"""
     for i, line in enumerate(lines):
         if line.startswith(comment):
-            if line.startswith((comment + ' %', comment + ' ?', comment + ' !')) and is_magic(line, main_language):
+            if line[len(comment):].lstrip().startswith(comment):
+                continue
+            if is_magic(line, main_language):
                 return False
             continue
         return i > 0 and _BLANK_LINE.match(line)

tests/test_cli.py

@@ -958,3 +958,12 @@ def test_warn_only_skips_incorrect_notebook(tmpdir, capsys):
 
     assert not os.path.exists(incorrect_md)
     assert os.path.exists(correct_md)
+
+
+@pytest.mark.parametrize('fmt', ['md', 'Rmd', 'py', 'py:percent', 'py:hydrogen'])
+def test_339_ipynb(tmpdir, fmt):
+    tmp_ipynb = str(tmpdir.join('test.ipynb'))
+    nb = new_notebook(cells=[new_code_cell('cat = 42')])
+    nbformat.write(nb, tmp_ipynb)
+
+    assert jupytext([tmp_ipynb, '--to', fmt, '--test-strict']) == 0