support optimistic locking with ETags and If-Match in rest server

currently it is not possible to do optimistic locking using the ETag feature in the REST server, because the "If-Match" HTTP header is ignored.

We allow to put a value many times with the same "If-Match" header, but according to the spec:

"A request intended to update a resource (e.g., a PUT) MAY include an If-Match header field to signal that the request method MUST NOT be applied if the entity corresponding to the If-Match value (a single entity tag) is no longer a representation of that resource. This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge. "

(copied from http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.24)