Adding accept header to callsites within SignalR

[MattyLeslie]

Adding accept header to callsites within SignalR

Summary of the changes
As mentioned in #47398, many of the HttpClient call sites within the SignalR library were missing their appropriate accept headers. This change aims to address that issue.

Description
Changes made include:

1. Adding accept header to NegotiateAsync() within HttpConnection to ensure header is present for negotiation requests.
2. Adding accept header to SendMessage() within SendUtils for the send requests.
3. Adding accept header to Poll() within LongPollingTransport for the get request.
4. Test to ensure first request and subsequent polling requests for LongPollingTransport have the correct accept header
5. Test to ensure the request for ServerSentEventsTransport has the correct accept header
6. Test to ensure the negotiate in HttpConnection has the correct accept header.

Fixes #47398 by ensuring the negotiation requests and other HttpClient call sites of SignalR are made with the relevant accept header.

[BrennanConroy]

Are you actually changing anything, or just kicking the build? If it's kicking the build please stop.
Also, could you please look into adding tests, see

aspnetcore/src/SignalR/common/Http.Connections/test/ServerSentEventsTests.cs

Line 34 in 42bc1a1

Assert.Equal("text/event-stream", context.Response.ContentType);

as an example.

[BrennanConroy]

Looks like you're missing negotiate? Otherwise I think that covers all the spots.

[MattyLeslie]

Thank you very much for the insight @BrennanConroy. I believe I have completed all the necessary tests.

[BrennanConroy]

Just some cleanup comments

[MattyLeslie]

@BrennanConroy, it seems functional tests from MVC are failing due to httpClient.timeout. Can I assume this is network related ?

[BrennanConroy]

Ignore it, I'll rerun the tests if I don't have any more comments after reviewing later today.

[BrennanConroy]

Thanks @MattyLeslie! Sorry for all the nit-picky comments 😄