Adding TextPlainFormatter to always handle returning strings as text\pla...

One Functional test will be added after #849
Couple of things to note here:
1. When the value is `null`, I am not sure if we should be explicitly setting the `Content-Length` header to `0` as otherwise the host layers might think that you are sending a chunked response...
2. In Web API, we prevent double buffering for scenarios where we can already calculate the length of the response content and as part of this `StringContent` is not buffered again as we can calculate the length of it...so need to see how its going to be in our case...

yeah, both these above points depend upon the response buffering story that is yet to be decided i think..

/cc: @yishaigalatzer