Fixing Content-Type bug for HTTP/1.1 POST Unary requests in Unity

[ShikiGami]

As described in #39 , there is a bug when trying to create Unary requests from Unity.
At the server side, the request content type will appear as application/octet-stream instead of the expected application/grpc-web which will not allow the GrpcWebSocketBridge middleware to execute properly because of this condition:

GrpcWebSocketBridge/src/GrpcWebSocketBridge.AspNetCore/GrpcWebBridgeMiddleware.cs

Line 25 in 8e37d22

if (!HttpMethods.IsPost(context.Request.Method) || context.Request.ContentType != "application/grpc-web")

The reason for this problem seems to be the implementation of the HTTP handler set for versions higher than Unity 2018.1, that uses UnityWebRequestHttpHandler.

In the implementation of UnityWebRequestHttpHandler, the Content Type seems to be set by the request accept header, but at no time in the creation of the request this header is set, instead the correct Content Type is set on the request content header "Content-Type".

I changed the implementation of UnityWebRequestHttpHandler to use this header.

[mayuki]

Thanks! Looks good to me. 👍