Allow cacheKeyWillBeUsed to influence the request method check

[markbrocato]

Fixes #2615

Moves the request method check after the call to cacheKeyWillBeUsed so that developers can potentially cache post requests as get requests.

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[markbrocato]

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[jeffposnick]

Thanks for the contribution! As mentioned in #2615, I'm not sure that I want to advertise this as a way to cache arbitrary POST requests, but it does makes sense to perform the method check on the cacheKeyWillBeUsed return value.

If folks who know what they're doing use that as a way to cache POST requests, then so be it...