Fixup method to update persisted keychain credential
Fixup method to update persisted keychain credential

The removed call to delete any existing credential
actually prevented updating any credential. It
also somehow prevented, possibly due to a race
condition, the deletion prevents the proper
subsequent addition of the same credential. 

Ultimately, this led to a persisted account
requiring re-authentication upon being loaded from
disk the second time.
tomlongo/nationbuilder-ios,nationbuilder/nationbuilder-ios