Fix issue with printing description of actual objects.

For objects like NSDictionary that have multiline descriptions, when the argument match failed, the failure message mangled the description of the actual object, squashing it into a single string with '\n' in place
of the newlines.

It looks like the issue was that description was called on the actual object before passing into the SBLArgumentMatcherResult. That was done as part of the refactoring in 38a50c1b, but I can't see any reason for it, and reverting it here doesn't break any other tests.