* lib/net/imap.rb: Fix undefined variable usage & refactor/DRY
  code.  Patch by @aledovsky. [Fixes GH-770]

* test/net/test_imap.rb: related test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48728 b2dd03c8-39d4-4d8f-98ff-823fe69b080e