Fix multiple value bug in html.parse_html_dict()

It is possible that a key in a MultiValueDict has multiple values, lists
are represented this way. When accessing a key in a MultiValueDict
it only returns the last element of that key. This becomes a problem
when parsing an html dict with a list inside of it.

To fix this problem we have to get and set the value using .getlist()
and .setlist().

This commit solves a bug that caused a list in a nested serializer to
not being properly parsed if it was sent as multipart formated data.

rest_framework/utils/html.py

@@ -80,10 +80,12 @@ def parse_html_dict(dictionary, prefix=''):
     """
     ret = MultiValueDict()
     regex = re.compile(r'^%s\.(.+)$' % re.escape(prefix))
-    for field, value in dictionary.items():
+    for field in dictionary:
         match = regex.match(field)
         if not match:
             continue
         key = match.groups()[0]
-        ret[key] = value
+        value = dictionary.getlist(field)
+        ret.setlist(key, value)
+
     return ret