net/smtp: add Noop function

[cryptix]

Hi,

I want to add the NOOP command to the smtp client.

I think it would be good to have this to check the status of a client connection.

It looks like a simple addition and I'd be willing to contribute this myself.

One thing I'm not sure about is how to test this. The examples use smtp.example.com but I don't think this is really functional?

[gopherbot]

Change https://golang.org/cl/71650 mentions this issue: net/smtp: added Noop to Client

[ianlancetaylor]

For testing, see net/mail/smtp_test.go and the serverHandle function.

[ianlancetaylor]

Sorry, I said net/mail/smtp_test.go but of course I meant net/smtp/smtp_test.go.