couchdb: fix TestSecurity on Go 1.7

In Go 1.7, where ResponseRecorder.Result was introduced, ContentLength
isn't handled properly by Result. Just remove the extra check for it in
the client code, and rely on io.EOF instead. This more robust anyway.

couchdb: fix options encoding

JSON keys are now handled explicitly, which is how CouchDB
does it internally. This means that fewer types are supported
(e.g. nil is always rejected if the option is not a known JSON option).