transport: Make error-handling for bad HTTP method consistent between HTTP/2 server transport and handler server transport

[jhump]

This tweaks transport.NewServerHandlerTransport so that it behaves more like the normal HTTP/2 transport returned by transport.NewServerTransport with regard to requests with the wrong HTTP method. In particular, it now sends back a 405 Method Not Allowed status code and correctly sets an Allow response header.

I've also moved the HTTP version check to after we've confirmed that this is a gRPC request (via content-type check) and changed that to also use a more appropriate HTTP status code: 505 HTTP Version Not Supported. (This is not really related to consistency since the other transport never has to worry about HTTP 1.1 requests. But it makes the checks a little more coherent since IMO it seems more appropriate to reject the HTTP version only once we've confirmed that it's a gRPC request.)

RELEASE NOTES:

• server: Fix some error cases when using a grpc.Server as an http.Handler with the Go stdlib HTTP server.

[codecov]

Codecov Report

Merging #6989 (d58a618) into master (3c2a44d) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6989      +/-   ##
==========================================
- Coverage   82.30%   82.20%   -0.10%     
==========================================
  Files         296      296              
  Lines       31454    31455       +1     
==========================================
- Hits        25888    25858      -30     
- Misses       4502     4525      +23     
- Partials     1064     1072       +8     

Files	Coverage Δ
internal/transport/handler_server.go	85.95% <100.00%> (+1.42%)	⬆️

... and 14 files with indirect coverage changes

[zasweq]

LGTM