Refactor Mail::Sendmail specs:

* Specify class name for top level describe block;
* Use before {}, same as before(:each) {};
* Refactor all similar or identical mail building code with let()
  helper;
* Set :sendmail delivery_method in global before block, as it was
  currently called in every single examples;
* Use `described_class' instead of hard coded class name;
* Remove the word `should' from all example descriptions;
* Use context instead of describe since we aren't really unit-testing;
* Fix wording in few example descriptions;
* Remove all "dead" code unrelated to the subject under test, or
  irrelevant for the concerned example.

Few minor changes:

* Fix inconsistencies in ruby quotes usages (' vs ");
* Fix long lines;
* Fix inconsistencies in blank line usages;
* Remove trailing spaces.
Remove unintented test setup in Mail::Sendmail specs
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.
Add spec on sendmail delivery arguments setting
Add spec on asking sendmail to stop flags reading

  Feature was introduced in commit
a944f31cf76961b685f1db329d20090b7f55cc82 but didn't had a spec.
Remove exim delivery method:

  All sendmail-compatible MTA (exim, but also many others) will work
without any problem with the sendmail delivery method. Implementing a
delivery method for each MTA, using a "non-standard" way, while we could
use the "standard sendmail" way will result on code duplication and
increase maintenance cost. It might also confuses users, making them
think a specific delivery method is required for Exim, while the
sendmail method works as expected.
Revert safe and working shell escaping in sendmail delivery

  Commit 81e9940b0e7a8140ea8178ced03c071c91f756e9 introduced a broken
and less safe escaping mechanism for sendmail command arguments, this
change revert a version more similar as it was just before. Arguments
may be quoted or escaped, but not both.
  We remove example verifying that destinations are quoted, but many
other examples need to be changed back to prior expectations (without
quotes), as they were changed by 81e9940b to force those other tests to
pass.