crypto/tls: server accepts RC4 by default

[anacrolix]

An SSL scan of my Go server with TLS enabled reports

This server accepts the RC4 cipher, which is weak. Grade capped to B.

I wonder if perhaps this one shouldn't be in the default set of cipher suites?

[minux]

Last time I checked, RC4 is still included in server cipher suite of nearly all major websites.

[titanous]

RC4 is prohibited by RFC 7465 and should not be used.

[adg]

cc @agl

[agl]

https://go-review.googlesource.com/#/c/7647

[anacrolix]

Cheers!