Prefer an implicit codec when using the gRPC default

Some gRPC servers in the wild appear to only accept requests when the
Content-Type subtype codec name is empty, implying the default of
"proto". For example Google Cloud API endpoints appear to require
Content-Type to be exactly `application/grpc` and disallow the
equivalent explicit form `application/grpc+proto`, rejecting requests
using the latter form by responding with a 404 and
`Content-Type: text/html`.

It's unclear if this is a load balancer (mis)configuration issue, or a
lack of support for specifying a codec in the server implementation, but
the end result is the same.

Seeing as the subtype qualifier is optional for gRPC, let's try to prefer
the implicit form when it is equivalent to the selected codec.

Note that this change only impacts gRPC. Elsewhere we continue to prefer
being explicit. This change should not impact servers.

protocol_grpc.go

@@ -846,6 +846,10 @@ func grpcContentTypeFromCodecName(web bool, name string) string {
 	if web {
 		return grpcWebContentTypePrefix + name
 	}
+	if name == codecNameProto {
+		// for compatibility with some servers prefer an implicit default codec
+		return grpcContentTypeDefault
+	}
 	return grpcContentTypePrefix + name
 }