If response map has content-type set, don't override or something

[Deraen]

Sometimes user needs to return json as text/plain type (because IE9).

This works:

(-> (ok (cheshire.core/generate-string {:ping "pong"}))
    (content-type "text/plain"))

Source: https://github.com/metosin/ring-middleware-format/issues/5

[Deraen]

Somewhat related: https://github.com/ngrunwald/ring-middleware-format/commit/6f2930789c1bf43c4001bc4fb19fcab284884a99 and https://github.com/ngrunwald/ring-middleware-format/issues/32