test: http_admin: fix wrong pack of ack_expectations[uuid]

ack_expectations[uuid] needs an object instead of a number.

It's a long time ago change since commit 7b52a05("Fix log, refactor
get_durability, fix switch primary"), however that commit forgot to
change it on http_admin.py side, thus introduced following error
while running regress-852 test:

    Traceback (most recent call last):
      File "$RETHINKDBtest/regression/issue_852.py", line 41, in <module>
        affinities = {dc: 1}, ack_expectations = {dc: 2})
      File "$RETHINKDB/test/common/http_admin.py", line 562, in add_namespace
        info = self.do_query("POST", "/ajax/semilattice/%s_namespaces/new" % protocol, data_to_post)
      File "$RETHINKDB/test/common/http_admin.py", line 349, in do_query
        return self.do_query_specific(host, http_port, method, route, payload)
      File "$RETHINKDB/test/common/http_admin.py", line 361, in do_query_specific
        return json.loads(self.do_query_specific_plaintext(host, http_port, method, route, payload, headers))
      File "$RETHINKDB/test/common/http_admin.py", line 374, in do_query_specific_plaintext
        raise BadServerResponse(response.status, response.reason)
    http_admin.BadServerResponse: Server returned error code: 400 Bad Request

And you can see following error from rethinkdb log:

    info: HTTP request throw a schema_mismatch_exc_t with what = Expected object instead got: 2

This patch does make the "400 Bad Request" error dispear, but the
testcase still failed due to rethinkdb crash.

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>