net/http: change Transport.Dialer to Transport.DialContext
net/http: change Transport.Dialer to Transport.DialContext

New in Go 1.7 so still possible to change.
This allows implementations not tied to *net.Dialer.

Fixes #15748.

Change-Id: I5fabbf13c7f1951c06587a4ccd120def488267ce
Reviewed-on: https://go-review.googlesource.com/23489
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

ImJasonH/go,codestation/go,derekmarcotte/go,polyverse-security/go,goodlang/good,linux-on-ibm-z/go,akutz/go,ImJasonH/go,nawawi/go,linux-on-ibm-z/go,CAFxX/go,4ad/go,akutz/go,goodlang/good,ImJasonH/go,vsekhar/elastic-go,vsekhar/elastic-go,suncycheng/go,polyverse-security/go,josharian/go,4ad/go,goodlang/good,golang/go,4ad/go,nawawi/go,cloudflare/go,CAFxX/go,theif519/go,polyverse-security/go,akutz/go,theif519/go,polyverse-security/go,nawawi/go,codestation/go,momchil-velikov/go,golang/go,nawawi/go,derekmarcotte/go,linux-on-ibm-z/go,josharian/go,golang/go,theif519/go,suncycheng/go,momchil-velikov/go,josharian/go,Cofyc/go,4ad/go,josharian/go,4ad/go,derekmarcotte/go,cloudflare/go,cloudflare/go,derekmarcotte/go,CAFxX/go,nawawi/go,christopher-henderson/Go,codestation/go,golang/go,suncycheng/go,momchil-velikov/go,ImJasonH/go,CAFxX/go,goodlang/good,goodlang/good,akutz/go,vsekhar/elastic-go,codestation/go,vsekhar/elastic-go,CAFxX/go,cloudflare/go,vsekhar/elastic-go,cloudflare/go,vsekhar/elastic-go,derekmarcotte/go,Cofyc/go,derekmarcotte/go,codestation/go,josharian/go,suncycheng/go,derekmarcotte/go,josharian/go,4ad/go,polyverse-security/go,codestation/go,linux-on-ibm-z/go,goodlang/good,4ad/go,linux-on-ibm-z/go,momchil-velikov/go,polyverse-security/go,cloudflare/go,codestation/go,linux-on-ibm-z/go,Cofyc/go,theif519/go,linux-on-ibm-z/go,golang/go,codestation/go,akutz/go,ImJasonH/go,Cofyc/go,akutz/go,nawawi/go,josharian/go,suncycheng/go,akutz/go,golang/go,momchil-velikov/go,momchil-velikov/go,ImJasonH/go,suncycheng/go,4ad/go,Cofyc/go,theif519/go,vsekhar/elastic-go,goodlang/good,Cofyc/go,josharian/go,CAFxX/go,theif519/go,Cofyc/go,theif519/go,derekmarcotte/go,polyverse-security/go,golang/go,golang/go,ImJasonH/go,theif519/go,Cofyc/go,nawawi/go,linux-on-ibm-z/go,ImJasonH/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,suncycheng/go,cloudflare/go,CAFxX/go,polyverse-security/go,goodlang/good,momchil-velikov/go,vsekhar/elastic-go,suncycheng/go,momchil-velikov/go