Fixed #5076 -- Properly decode POSTs with non-utf-8 payload encoding

Thanks daniel at blogg.se for the report and Aymeric Augustin for
his assistance on the patch.

django/core/handlers/wsgi.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+import codecs
 import logging
 import sys
 from io import BytesIO
@@ -144,6 +145,14 @@ class WSGIRequest(http.HttpRequest):
         self.META['PATH_INFO'] = path_info
         self.META['SCRIPT_NAME'] = script_name
         self.method = environ['REQUEST_METHOD'].upper()
+        _, content_params = self._parse_content_type(self.META.get('CONTENT_TYPE', ''))
+        if 'charset' in content_params:
+            try:
+                codecs.lookup(content_params['charset'])
+            except LookupError:
+                pass
+            else:
+                self.encoding = content_params['charset']
         self._post_parse_error = False
         try:
             content_length = int(self.environ.get('CONTENT_LENGTH'))
@@ -155,6 +164,21 @@ class WSGIRequest(http.HttpRequest):
     def _is_secure(self):
         return 'wsgi.url_scheme' in self.environ and self.environ['wsgi.url_scheme'] == 'https'
 
+    def _parse_content_type(self, ctype):
+        """
+        Media Types parsing according to RFC 2616, section 3.7.
+
+        Returns the data type and parameters. For example:
+        Input: "text/plain; charset=iso-8859-1"
+        Output: ('text/plain', {'charset': 'iso-8859-1'})
+        """
+        content_type, _, params = ctype.partition(';')
+        content_params = {}
+        for parameter in params.split(';'):
+            k, _, v = parameter.strip().partition('=')
+            content_params[k] = v
+        return content_type, content_params
+
     def _get_request(self):
         if not hasattr(self, '_request'):
             self._request = datastructures.MergeDict(self.POST, self.GET)