Fixing content-security-policy for IE 10 / 11

This fixes https://github.com/evilpacket/helmet/issues/42.
Platform.js has a bug where it was mis-detecting IE11 as Firefox, that has a PR here: https://github.com/bestiejs/platform.js/pull/57

This includes code to detect IE11 correctly until the platform.js PR is merged in (alternatively, you can wait for that request to be released and bump the platform.js version).

This PR also includes code that fixes some typos with CSP test suite, headers are sent back lowercased so the test `assert(res.header['X-Content-Security-Policy'] === undefined);` would always return true.