Extract & Wrap PropertyValueConverter method to fix tests

Unable to access PropertyValueConverterResolver.Current from a test, so wrapping in a quick check for UmbracoContext.  Probably not the best fix, but wasn't sure best way or if overkill to create an abstraction.

Also extracted the logic that finds/executes PropertyValueConverters to make testing easier.
Merge pull request #95 from imulus/fix/tests

Fix Fieldset tests