Fix duplicate zero= entries for X-Analytics

Per Bug 62922 - previous change introduces duplicate "zero=..."
values into X-Analytics header.
I4fe500ce34973df63fb67dc22b0ffa1b3d3b0557 introduced this bug,
and I suspect it is due to the cached object being appended with
an extra value. Once the cache clears, this check could be removed.

Change-Id: I3781a64481ecab21aec78f382c33a592fa679955