Made the Negotiator awaitable

[thecodejunkie]

Prerequisites

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• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for your change (where applicable)

Description

Enables the use of Negotiator from inside an async module by calling await on it. This approach still enables the non-await call to it from inside LegacyNancyModule

public class TestModule : NancyModule
{
    public TestModule() : base("asyncnegotiator")
    {
        Get["/"] = async (parameters, ct) =>
        {
            return await Negotiate
                .WithModel(new RatPack { FirstName = "Nancy " })
                .WithMediaRangeModel("text/html", new RatPack { FirstName = "Nancy fancy pants" })
                .WithView("negotiatedview")
                .WithHeader("X-Custom", "SomeValue");
        };
    }
}

[thecodejunkie]

Just realized that this will automatically solve #2421 as well, since View["..:"] ultimately returns a Negotiator instance as well 😛

[grumpydev]

Cool

[thecodejunkie]

@NancyFx/most-valued-minions ⌚

[thecodejunkie]

Replaced by #2441