fix multipart decoder to allow no-contents part (fixes requests#352)

requests_toolbelt/multipart/decoder.py

@@ -59,7 +59,18 @@ def __init__(self, content, encoding):
             first, self.content = _split_on_find(content, b'\r\n\r\n')
             if first != b'':
                 headers = _header_parser(first.lstrip(), encoding)
+        # because pre-split by boundary was done,
+        # having only a single newline and the '--'
+        # means that this is a 'no content' part
+        elif content.endswith(b'\r\n') and not content.endswith(b'\r\n\r\n'):
+            self.content = None
+            headers = _header_parser(content.strip(), encoding)
+            if not headers:
+                raise ImproperBodyPartContentException(
+                    'No contents part without any header is invalid.'
+                )
         else:
+            print("==" + content.decode() + "===")
             raise ImproperBodyPartContentException(
                 'content does not contain CR-LF-CR-LF'
             )
@@ -68,6 +79,8 @@ def __init__(self, content, encoding):
     @property
     def text(self):
         """Content of the ``BodyPart`` in unicode."""
+        if self.content is None:
+            return None
         return self.content.decode(self.encoding)