Fix content-type of JSON API responses to application/json

An MTA Bus Time API user [reported](https://groups.google.com/d/msg/mtadeveloperresources/XuCZw13v_T4/oGT9aHJIiU8J) that calls to the OneBusAway API were returning JSON responses with a `Content-Type` of `text/javascript`, rather than the expected `application/json`.  This was previously fixed by @barbeau for core OBA in onebusaway/onebusaway-application-modules@78c5067, but the same fix needs to be applied to OBANYC, where the affected module has been renamed `onebusaway-nyc-api-webapp`.