Set `cache_key` correctly for Rails 4

[alindeman]

It seems like we're doing something wrong with cache_key in view specs? rails/rails#13208 (comment)

Investigate and fix as required. Might be able to revert #864

• 2.14
• 2.99
• 3.0

/cc: @JonRowe

[JonRowe]

As far as I can see it's all internal to the Rails side of the test helpers.

[JonRowe]

I think it's still worth blowing away the cache inbetween tests, we'd never intentionally want that behaviour.

[alindeman]

Clearing the cache is fine as far as I can tell. Reopen or ping on this issue if that ever changes.