method GetPostContent don´t permit too long uri

[alexandercamps]

There is limitation in class FormUrlEncodedContent about length information sended, in a post HTTP request. In my case I make a post sending ~200 Kb, but the class FormUrlEncodedContent don´t allowed. So I needed change method GetPostContent , changing the class FormUrlEncodedContent by StringContent and created the request on POST Uri protocol.