Add Content-Type header

[jellybob]

This patch adds a Content-Type header to redirects, since without one Rack::Lint will throw an exception.

[mguterl]

@tylerhunt any chance on getting this merged?

[tylerhunt]

Seems like a blank Content-Type header and empty body is more common for redirects.

• https://github.com/6twenty/rack-static_fallback/commit/26094e0e2701a3bdd3dacb18f997b4a9d130d1f7
• eric1234/rack_mailer@9f31142

[mguterl]

@tylerhunt that makes sense. as long as it passes Rack::Lint so I can run in development I'm happy.