Android: Modify native HTTP handling to only cache "GET" responses

Summary:
Titanium's custom TiResponseCache.java class currently caches all HTTP responses and ignores the HTTP method. Meaning that it'll cache "POST", "PUT", "DELET", etc. which is a problem when using REST web APIs.

It should be changed to only cache HTTP "GET" responses.
(Note: iOS does not cache "HEAD" responses.)

Note:
This is not an issue with Titanium's Ti.Network.HTTPClient on Android since it's currently set up to never cache responses. The only Titanium JavaScript API which does HTTP response caching is when you load images via URLs, which are "GET" requests.

The above is only an issue for app's which do HTTP requests natively in Java, such as via a 3rd party native library.