Connection: add leading zero to single digit date

[AlexandrBoltris]

yandex.ru imap server does not accept new messages with customized arrival date.

Problem looks like this:

=> 'A6 APPEND "test2" "7-Nov-2016 18:49:23 +0200" {160+}'
=> 'From: Fred Foobar <foobar@Blurdybloop.COM>[skipped]
<= 'A6 BAD Command syntax error. sc=jwOWLg0ZsiE1_221858_22j'

Message can be accepted after adding a leading zero "7-Nov-2016" -> "07-Nov-2016".

=> 'A6 APPEND "test2" "07-Nov-2016 18:49:23 +0200" {160+}'
=> 'From: Fred Foobar <foobar@Blurdybloop.COM>[skipped]
<= 'A6 OK [APPENDUID 1506098273 40] APPEND Completed.'