Fix output formatting differences between JRuby and Ruby when converting Float to JSON

[evaniainbrooks]

issue #817

[evaniainbrooks]

It appears this needs to wait for a new version of JRuby 9.4 to be released.

[headius]

We will need to patch this to use the old logic on older JRuby versions or it will break for users on any version other than 10.0.3.0. I will suggest a change.

[evaniainbrooks]

@headius I don't mind following up on this again. If you have a moment, let me know what you want to see and I will give it a go.

• In a static initialization block, set a flag/MethodHandle to switch on the new implementation
• Use a ThreadLocal scratch buffer for the tmp ByteList instead
• Optimize the old path, remove some uncessary allocations