gh-137 Compression tests should not test for content-length

* Improved test verticle to look for test methods in parent class too
* This makes subclassing for compression tests easier and makes it
more straightforward to run a single test, rather than one after the
the other, which gets messy with test completion if one of the
iterations fails.
gh-136 Expose Scala collections for SharedData and multi maps

* Avoid leaking java or Vert.x core collections to Scala clients
* Update javadoc to warn of conversions