net/http: racing write to t.ProxyConnectHeader in dialConn when proxy URL includes auth credentials [1.13 backport]

[gopherbot]

@bcmills requested issue #36431 to be considered for backport to the next 1.13 minor release.

@gopherbot, please backport to Go 1.13 and 1.12.

This is a data race in net/http, and the fix is straightforward.

[gopherbot]

Change https://golang.org/cl/213657 mentions this issue: [release-branch.go1.13] net/http: avoid writing to Transport.ProxyConnectHeader

[gopherbot]

Closed by merging 367da16 to release-branch.go1.13.