Specify Content-Type as HTML in rack developer mode.

I was using developer mode in Chrome. Chrome rendered the page as plain text, but Safari rendered it as HTML. I curled `/newrelic`, and noticed `Content-Type` was missing. I dug around and found that [in Rack 1.5, the `Content-Type` in Rack responses does not default to text/html] (https://github.com/rack/rack/commit/3623d04526b953a63bfb3e72de2d6920a042563f#L2L24). So to be consistent, `Content-Type` should be explicitly set.