[ruby-core:26150] Patch: Let Net::SMTP continu when at least one RCPT is accepted

Hi all,

While writing an SMTP daemon in Ruby, one of my tests
(that uses Ruby's existing Net::SMTP library) failed.
I was testing that my daemon will deliver mail when
only some RCPTs specified by the client were accepted.

Ruby's Net::SMTP quits when the first Unknown User is
encountered. I think Net::SMTP should continue with
other RCPTs, then use DATA to let the mail be delivered
to existing users.

I think this should be so, because
 - RFC821 mentions clearly in the second paragraph in section 2
   "if not, it responds with a reply rejecting that recipient
   (but not the whole mail transaction)", and
 - RFC2821 leaves this in the middle, but still gives
   an example in section  D.1 A Typical SMTP Transaction Scenario
   "C: RCPT TO:<[e-mail]>
      S: 250 OK
      C: RCPT TO:<[e-mail]>
      S: 550 No such user here
      C: RCPT TO:<[e-mail]>
      S: 250 OK"

So, I wrote code to accomplish this.
Please find a patch attached.

Comments welcome, especially on the Errors constructed and returned.
They are in plain text; I can imagine making the unknown and ok
users explicit (arrays within the exception), so that code that uses
Net::SMTP has an easier time determining which emails were not sent.

Bye,
Kero.

PS: patch applies to both ruby_1_8 branch and trunk
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