Static JavaProxy#getJavaClass needs to clear $!

Calls from Ruby which go through JavaProxy.getJavaClass (ie.
field_writer, field_reader, and field_accessor) can result in $!
incorrectly containing a NoMethodError from its attempt to invoke
"java_class".  Ensure we clear it in the catch block.
Merge pull request #1636 from dmarcotte/test-clear-last-err

Static JavaProxy#getJavaClass needs to clear $!