Use response.ContentType when rendering BinaryResult

Patch from:
https://github.com/revel/revel/commit/58a6b63875adc1f4c2844eec4f516e1cc0801e1f
Set static content type more explicitly

Patch from:
https://github.com/revel/revel/commit/da4c1765b35368f2603fa0e8c84a09b5b2a85176