Fixed issue with duplicated Http Headers

When a header was having multiple values, the iOS solution was to pick the last, and the Android solution was to create multiple headers.

I believe the correct behavior is to separate each value with a comma. 

**Example:**

In the current implementation I was getting `400 - Bad Request` responses because of the headers being sent from my Android device looked like this:

	User-Agent: Android/4.1.1
	User-Agent: Music.Droid.Msh/1.0.0.0
	User-Agent: Rev/local
	User-Agent: E247.Api.Client/0.1.0.157
	User-Agent: (Google Nexus 4 - 4.1.1 - API 16 - 768x1280 vbox86p)
	Accept-Language: en-US
	Accept-Encoding: gzip
	Accept-Encoding: deflate

With the suggested changes the headers would render like this:

	User-Agent: Android/4.1.1,Music.Droid.Msh/1.0.0.0,Rev/local,E247.Api.Client/0.1.0.157,(Google Nexus 4 - 4.1.1 - API 16 - 768x1280 vbox86p)
	Accept-Language: en-US
	Accept-Encoding: gzip,deflate