Handle encoding of None when decoding unicode

If encoding is None, decoding will throw the following TypeError:
TypeError: unicode() argument 2 must be string, not None

If this is the case, attempt to run without any set encoding

requests/models.py

@@ -834,6 +834,11 @@ class Response(object):
             #
             # So we try blindly encoding.
             content = str(self.content, errors='replace')
+        except TypeError:
+            # A TypeError can be raised if encoding is None
+            #
+            # So we try blindly encoding.
+            content = str(self.content, errors='replace')
 
         return content