Fix for newer version of httplib

httplib in python 2.7 requires that the lenght parameter is a string
and not an int.

tweepy/api.py

@@ -747,7 +747,7 @@ def _pack_image(filename, max_size):
         # build headers
         headers = {
             'Content-Type': 'multipart/form-data; boundary=Tw3ePy',
-            'Content-Length': len(body)
+            'Content-Length': str(len(body))
         }
 
         return headers, body