implement put data-present parameter in http servant

implement put data-present parameter in http servant

Changed the protocol docs because servant parses "true" and "false" for
booleans in query parameters, not "1" and "0".

clientPut with datapresent=True is not used by git-annex, and I don't
anticipate it being used in git-annex, except for testing.

I've tested this by making clientPut be called with datapresent=True and
git-annex copy to a remote succeeds once the object file is first
manually copied to the remote. That would be a good test for the test
suite, but running the http client means exposing it to at least
localhost, and would fail if a real http client was already running on
that port.