Add separate set methods to set common http headers in http requests

[unsuitable001]

HttpHeaders class also gives us some handy methods to mutate some commonly used headers (such as contentLength, persistentConnection etc.) besides the set method.

set method can be used to add any headers, including but not limited to the above mentioned ones. set method is being implemented via #26 and tracked using #25.

This issue is a superset/improvement of #25.