Accept https proxy

[philipatl]

Fixes #739

Summary of Changes

1. call proxy_RegisterDialerType for the https scheme to allow communication to https proxies
2. some scheme-specific setup is necessary in Dialer's DialContext in order to use the TLSClientConfig
3. add a UsesTLS method to the proxy_Dialer interface to ensure the https forwardDialer can inspect that https is supported, and to adjust if necessary

[ghost]

I am not a maintainer of the project, but I do have some comments on the CL:

• The CL modifies the generated file x_net_proxy.go. Avoid editing the file or regenerate the file if a new version of x/net/proxy has the desired functionality.
• The CL is missing tests.

[philipatl]

I am not a maintainer of the project, but I do have some comments on the CL:

• The CL modifies the generated file x_net_proxy.go. Avoid editing the file or regenerate the file if a new version of x/net/proxy has the desired functionality.
• The CL is missing tests.

Thanks, I've made the suggested changes.

[philipatl]

Is there anything else needed here that prevents merging? Any comments? @garyburd

[joemiller]

It would be great to see https proxy support. Are there any open blockers or concerns that the community could help with this PR or an alternate implementation?

[kamikazechaser]

Are there any open blockers or concerns that the community could help with this PR or an alternate implementation?

Almost every PR is blocked by a lack of an active maintainer.

[tomerb-neosec]

Why this PR wasn't merged?

[AlexVulaj]

@tomerb-neosec it looks like there are merge conflicts in this PR. If you get those taken care of we can go forward with reviewing and getting this merged in. Thanks for your contribution!