Set Content-Type header before headers are written
Set Content-Type header before headers are written

* It's not possible to change headers after writeHeader or write

[#136945657]

Signed-off-by: Ed King <cb0ef010d3b7764b76e01d19f7526ae755120a2b@pivotal.io>

cloudfoundry-incubator/garden,cloudfoundry-incubator/garden