Fix error when Accept header is specified more than once

Currently, when more than one Accept header field is present in the request, it's causing a 500 Internal Server Error to be returned.

Real world example:
Googlebot (as tested from "Fetch as Google" in Webmaster Tools) is setting the Accept request header twice, and this error is causing Google to think my site is unavailable.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2 allows for header fields to be set more than once (read the entire last paragraph under "4.2 Message Headers"), so this is a case that should be handled without error.