PostFormat<> calls without a decoder

[frs3]

There are several PostFormat<> class that have an empty decoder, which causes an exception. e.g.

_caller.PostFormat(rawXml, HttpContentTypes.TextPlain, string.Empty, "/app/rest/buildTypes/{0}/steps", locator);

Fix:

_caller.PostFormat(rawXml, HttpContentTypes.ApplicationXml, "/app/rest/buildTypes/{0}/steps", locator);

[dan-turner]

Friendly tip for next time @frs3, if you'd implemented the fix yourself 5 months ago it would have been fixed for me just now :). I also wouldn't have had to go digging through github issues for existing bug report when I found it too.