Detect MultiDict when generating json body

Detects webob.multidict Multidict at sanitizerand returns mixed()
out of them so they can be parsed as json.

Fixes image-list when using Image API v2 with registry.

Implements a test to verify that the /rpc endpoint exists as
intended.

Closes bug: #1246980

Change-Id: I3d28ca5d8f1af0ec7b9e7a8684e3160f674b8f4c
Merge "Detect MultiDict when generating json body"