Use bytes BOUNDARY on django < 1.5

Django's encode_multipart was updated in django 1.5 to work internally
with unicode and convert to bytes.

In django >= 1.5 we therefore need to pass the BOUNDARY as unicode. In
django < 1.5 we still need to pass it as bytes.

rest_framework/renderers.py

@@ -10,6 +10,7 @@ from __future__ import unicode_literals
 
 import copy
 import json
+import django
 from django import forms
 from django.core.exceptions import ImproperlyConfigured
 from django.http.multipartparser import parse_header
@@ -597,7 +598,7 @@ class MultiPartRenderer(BaseRenderer):
     media_type = 'multipart/form-data; boundary=BoUnDaRyStRiNg'
     format = 'multipart'
     charset = 'utf-8'
-    BOUNDARY = 'BoUnDaRyStRiNg'
+    BOUNDARY = 'BoUnDaRyStRiNg' if django.VERSION >= (1, 5) else b'BoUnDaRyStRiNg'
 
     def render(self, data, accepted_media_type=None, renderer_context=None):
         return encode_multipart(self.BOUNDARY, data)