Fix detection of HTML accept header

Fix detection of HTML accept header so that requests for non-HTML resources can still get through to the proxy.

[This commit](https://github.com/stefanpenner/ember-cli/commit/c7203999d33aca91cfe4394389c89efaeff9cb23) broke the `--proxy` feature because `req.accepts('html')` returns true if the `accepts` header includes `*/*`, so my API requests no longer go through the proxy. The XHR requests to my API don't accept `text/html`, so previously they would go through the proxy to the API.