Fixes broken UNLOCK method

As far as I understand from the specs, lock token URI (which is often "opaquelocktoken" in many cases) is a part of lock token and should be provided to UNLOCK method along with the token. Milton tears it off and thus UNLOCK doesn't work with e.g. Apache (although seems to work with Milton server).

This change remains the contents of href tag as is and also adds surrounding <> which seem to be necessary in the header
See http://www.webdav.org/specs/rfc4918.html#METHOD_UNLOCK