Combine proper-message tests

I had needlessly duplicated the XML output for these tests. It's
much more clear this way.
Add dissoc-nil function/test

Strip keys with nil vals out of maps.
Add failing test for nil optional keys

If a required key has a nil value, clj-rss throws a reasonable
exception. If an optional key has a nil value, it throws the
cryptic "java.lang.NullPointerException". Instead, it should
just strip the offending optional keys beforehand.
Strip optional keys with nil vals from items

This adds a bit of tolerance for items that are not properly formed
without causing any other problems.
Clean up dissoc-nil

Add doc string and rename parameter for clarity.
Merge pull request #9 from holguinj/strip-nil

Strip optional keys with nil vals from items