Incomplete objects throw NoMethodErrors on decode

[nateberkopec]

   OkJson.decode("{")
   OkJson.decode("[")

Both throw NoMethodError rather than the expected OkJson::Error

[nateberkopec]

I couldn't figure out how to write a test for this using your test harness, but the way we fixed it over at raven-ruby is here.

[bfontaine]

I confirm the bug still exists.