Fixed #19101 -- Decoding of non-ASCII POST data on Python 3.

Thanks Claude Paroz.

django/http/multipartparser.py

@@ -110,7 +110,7 @@ class MultiPartParser(object):
         # HTTP spec says that Content-Length >= 0 is valid
         # handling content-length == 0 before continuing
         if self._content_length == 0:
-            return QueryDict(MultiValueDict(), encoding=self._encoding), MultiValueDict()
+            return QueryDict('', encoding=self._encoding), MultiValueDict()
 
         # See if the handler will want to take care of the parsing.
         # This allows overriding everything if somebody wants it.

django/http/request.py

@@ -276,6 +276,9 @@ class QueryDict(MultiValueDict):
             encoding = settings.DEFAULT_CHARSET
         self.encoding = encoding
         if six.PY3:
+            if isinstance(query_string, bytes):
+                # query_string contains URL-encoded data, a subset of ASCII.
+                query_string = query_string.decode()
             for key, value in parse_qsl(query_string or '',
                                         keep_blank_values=True,
                                         encoding=encoding):