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