Improve expectations in Mail::Sendmail specs:

  Many examples weren't only testing the specific feature described, but
relied on testing every arguments received by #call. In this change,
instead of testing all arguments, we test only specific argument(s)
related to the testing intent of the example.
  This prevent many test suddenly breaking when an unrelated part of
code is changed, and will also give more specific feedback on failure.
This should also ease maintenance or implementing new features by
avoiding the need to "hack" unrelated examples.
Revert safe shellescape method for Mail::Sendmail

FIXME: revert test about signaling end of options to sendmail

it 'asks sendmail to stop processing flags before giving destinations'