fix expire function for custom keys

[thstarshine]

should use key (derived from cacheKey function) instead of original path when doing all redis commands.

[reidab]

This looks like the most concise of the three PRs that fix expiration for custom cache keys. Any chance of getting it merged?

[Uysim]

@tomdale I think you should merge this as soon as possible

[benvoynick]

@tomdale Would appreciate a merge for this issue, as @reidab says this one seems the most concise of the several that have been submitted.