Preserve return value of with-doubles body

[fredZen]

Hi,

thanks for writing test-doubles. I'm surprised it isn't the number one result when searching for "clojure mock", it beats all the alternatives I've found.

This pull requests adds support for putting with-doubles inside the assertion rather than the other way around. Would you be interested in it? There are situations where I find this convenient, especially when using metosin's test-it:

(facts "about my function"
  (with-doubles ....) => some-result
  (with-doubles ...) => some-other-result)