[search] Support schemaless collections

[25/May/2016 18:14:35 -0700] middleware   INFO     Processing exception: unhashable type: 'list': Traceback (most recent call last):
  File "/home/romain/projects/hue/build/env/local/lib/python2.7/site-packages/Django-1.6.10-py2.7.egg/django/core/handlers/base.py", line 112, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/romain/projects/hue/build/env/local/lib/python2.7/site-packages/Django-1.6.10-py2.7.egg/django/db/transaction.py", line 371, in inner
    return func(*args, **kwargs)
  File "/home/romain/projects/hue/apps/search/src/search/decorators.py", line 44, in decorate
    return view_func(request, *args, **kwargs)
  File "/home/romain/projects/hue/apps/search/src/search/views.py", line 154, in search
    raise PopupException(e, title=_('Error while accessing Solr'))
PopupException: unhashable type: 'list'

[25/May/2016 18:14:35 -0700] resource     DEBUG    GET Got response: {"responseHeader":{"status":0,"QTime":1,"params":{"hl.fragsize":"1000","fl":"*","hl.snippets":"5","start":"0","user.name":"hue","q":"*:*","doAs":"romain","hl.fl":"*","wt":"json","hl":"true","rows":"10"}},"response":{"numFound":2,"start":0,"docs":[{"id":"change.me","titleddd":["change.me"],"_version_":1535351343319875584},{"id":"44","Artist":["Old Shews"],"Album":["Mead for Walking"],"Released":["1988-08-13T00:00:00Z"],"Rating":[0.01],"FromDistributor":[14],"Sold":[0],"_version_":1535351444544159744}]},"highlighting":{"change.me":{},"44":{}}}