Don't mutate JSON.dump_default_options from dump

The use of Hash#update from the JSON.dump method was mutating the
dump_default_options hash on any call to dump with a limit provided. An
individual method call with an overriding value shouldn't update the
defaults in this way.
Merge pull request #217 from jasonrclark/dont_mutate_dump_default_options

Don't mutate JSON.dump_default_options from dump