Fix a recursion problem with masked arrays in get_converter introduced in f8344a14504

If a masked array with the first element masked is passed in to get_converter, the   converter = self.get_converter(xravel()[0])  call causes a RuntimeError: maximum recursion depth exceeded.  This patch will pass the first unmasked value, or skip the call if all elements are masked.
Add a line to test masked arrays with the first element masked
Merge pull request #2290 from selasley/master

Fix a recursion problem with masked arrays in get_converter