Strip nil values from hashes before using them.

Parature does not accept nil values in the xsi format, so we strip them out otherwise an otherwise valid customer or ticket hash could be rejected for malformed XML.
Raise the actual XML error from Parature as an exception.

We now return the actual XML error from Parature so they can be fixed rather than the RestClient errors which are cryptic at best when working with Parature.
Merge branch 'master' of github.com:MatthewCallis/HotDogPrincess