Closed Issue: 405 response should include an "accept" header [1129]

According to the spec for 405 errors, [10.4.6](https://tools.ietf.org/html/rfc2616#page-66):
> The response MUST include an Allow header containing a list of valid methods for the requested resource.

WebAPI doesn't include this header today.
Comments: Verified. Checkin: a618c26a3682b774881648989f2680b754f1913e