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

One Functional test will be added after #849
Setting Content-Length is strongly preferred to chunked, if you can calculate it with a reasonable amount of effort. In this case, consider calling context.SelectedEncoding.GetByteCount and see how that affects your perf.  The result will vary widely depending on your selected encoding (ASCII = constant, UTF-8 = variable).