Unify response header handling with ring.util.response/get-header, fixes #50

This version of get header should be little bit more efficient than the one here https://github.com/PetrGlad/ring/commit/get-header/, on which it is based.

Also there is a blazingly fast, no-allocation, commented version of get-header, that will work on clojure 1.5, as soon as it is enabled via conditional compilation. The fact remains, however, that it is an O(n) case-insensitive string search. It shouldn't be a problem for reasonable counts of headers.

middleware.not-modified and middleware.content-type are updated to make use of get-header when interpreting response headers. Fixes #50