Several fields are incorrect when testcase is initialized from search

[lukaszachy]

Seems that fields (e.g. setup) which are not returned by TestCase.filter xmlrpc call are set to None but should be made 'unknown/unfetched' instead.

Compare output of

> import nitrate
> testcases = list(nitrate.TestCase.search(script='SOME_SCRIPT'))
> for tc in testcases:
      print(tc.setup)
None
None

With asking explicitly for fetch:

> for tc in testcases:
      tc._fetch()      
      print(tc.setup)
<p> Test </p>
<p> Test </p>

When TestCase is initialized via case_id then all fields work correctly.