Better test coverage

[mernen]

The library needs more extensive testing. Through bug reports on json-jruby, I've noticed the following parts are not covered:

• Bignums
• Non-UTF strings
• Strings with isolated surrogates (see issue json/ext handles isolated escaped surrogates inconsistently #38)
• Generator::State getters and setters (settings are always being passed on initialization)
• Strings whose internal buffer offset is nonzero (a rather specific implementation detail, but could be useful nonetheless), i.e.:
  str = 'xx["foobar"]'
  substr = str[2..-1]
  JSON.parse(substr) # confuses json-jruby 1.1.5

[hsbt]

We welcome to your patch for test coverage.