Fix a naming conflict jrubyc can introduce in generated java.

A ruby argument named 'result' will conflict with the stack local used for the return value: both will become 'ruby_result' in java.  Namespace arguments away from this by prefixing them with 'ruby_arg_' instead of merely 'ruby_'.

See #812
Merge pull request #894 from phs/phs/issue-812-jrubyc-variable-ruby_result-is-already-defined

Fix a naming conflict jrubyc can introduce in generated java.