Adapting testStatsKey Testcase to account changes in 2.1.0.

This changeset makes sure that the test case passes for both
post 2.1.0 and pre 2.1.0, because the exposed key stats
have changed between versions.

Change-Id: Ia32dd34c5cb5b504471f4660f152e7f4799b8f66
Reviewed-on: http://review.couchbase.org/27215
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>