net/http: for http2, use the priority write scheduler by default
net/http: for http2, use the priority write scheduler by default

Updates #18318

Change-Id: Ibd4ebc7708abf87eded8da9661378b5777b8a400
Reviewed-on: https://go-review.googlesource.com/43231
Run-TryBot: Tom Bergan <3b77d7da34779f829901a664ac7c87d255b041af@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

vsekhar/elastic-go,polyverse-security/go,akutz/go,golang/go,CAFxX/go,akutz/go,Cofyc/go,cloudflare/go,codestation/go,codestation/go,christopher-henderson/Go,polyverse-security/go,CAFxX/go,CAFxX/go,josharian/go,nawawi/go,derekmarcotte/go,polyverse-security/go,CAFxX/go,derekmarcotte/go,CAFxX/go,derekmarcotte/go,ImJasonH/go,akutz/go,codestation/go,ImJasonH/go,codestation/go,CAFxX/go,golang/go,akutz/go,polyverse-security/go,golang/go,golang/go,josharian/go,nawawi/go,Cofyc/go,josharian/go,CAFxX/go,ImJasonH/go,CAFxX/go,ImJasonH/go,vsekhar/elastic-go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,nawawi/go,golang/go,derekmarcotte/go,golang/go,codestation/go,Cofyc/go,cloudflare/go,derekmarcotte/go,ImJasonH/go,ImJasonH/go,vsekhar/elastic-go,nawawi/go,ImJasonH/go,derekmarcotte/go,akutz/go,josharian/go,ImJasonH/go,cloudflare/go,josharian/go,josharian/go,Cofyc/go,polyverse-security/go,cloudflare/go,vsekhar/elastic-go,Cofyc/go,akutz/go,josharian/go,Cofyc/go,derekmarcotte/go,cloudflare/go,polyverse-security/go,cloudflare/go,nawawi/go,vsekhar/elastic-go,akutz/go,codestation/go,vsekhar/elastic-go,vsekhar/elastic-go,derekmarcotte/go,golang/go,nawawi/go,Cofyc/go,Cofyc/go,polyverse-security/go,vsekhar/elastic-go,cloudflare/go,polyverse-security/go,golang/go,josharian/go,codestation/go,codestation/go