WA08: DefaultContentNegotiator.Negotiate: Remove Linq and foreach over List<T>.

WebAPI Async SelfHost is impacted by the overhead of Linq and foreach over List<T>. List<T> enumerators are 4.83% of allocations, and Linq helpers are 3.23% of exclusive time. These are reduced per request by this change. The Negotiate method is also very performance sensitive with 7.86% inclusive time.
Opt3B: PrefixContainer: Avoid array allocation.