Fix 500 error on overly-large API request body

Paster doesn't mind unicode responses, but gunicorn wants byte strings
only:

    Error handling request
    Traceback (most recent call last):
      File "/usr/lib/python2.7/dist-packages/gunicorn/workers/sync.py", line 131, in handle_request
        resp.write(item)
      File "/usr/lib/python2.7/dist-packages/gunicorn/http/wsgi.py", line 283, in write
        assert isinstance(arg, binary_type), "%r is not a byte." % arg
    AssertionError: u'{' is not a byte.

See https://gist.github.com/JordanMilne/4a7d1b5eb317e607479b for repro

r2/r2/config/middleware.py

@@ -317,7 +317,7 @@ def _wsgi_json(start_response, status_int, message=""):
         "error": status_int,
         "message": message
     })
-    return filters.websafe_json(data)
+    return [filters.websafe_json(data).encode("utf-8")]
 
 
 class LimitUploadSize(object):