[TIMOB-28060] Ti.API: Parity issue in HTTPClient response headers

There is a major parity issue when working with response headers in Titanium. On iOS, there is the [responseHeaders](http://docs.appcelerator.com/platform/latest/#!/api/Titanium.Network.HTTPClient-property-responseHeaders) property that returns a proper dictionary of all response headers. On Android, there is the [allResponseHeaders](http://docs.appcelerator.com/platform/latest/#!/api/Titanium.Network.HTTPClient-property-allResponseHeaders) property that returns all response headers combined as a string. That one should be deprecated and move to the more commonly used dictionary pattern (like other libraries do).