[ci skip]

This fixes the spec suite to expose issues with `ActionMailer` being
configured incorrectly when previews are not available.

This addresses several issues:

- The incorrect configuration options are only exposed once
  `ActionMailer::Base` is loaded; this forces the class to load in the
  mailer initializer and at the end of our script
- This adds specs specifically for the "development" environment which
  were missing before; it appeared that by not setting the environment
  it would default to "development" per the script, however, when the
  command shells out the `RAILS_ENV` is already set to "test" from rspec
  running.

  To be certain we cover the other possible edge cases we specifically
  add more tests for `ActionMailer` not being available, and for a
  custom configuration being set after `rspec-rails` is loaded.
- Despite us asking Rails to turn off eager loading in our custom
  script, several files were still getting eager loaded; this
  explicitly clears the eager load paths to stop this
- This fixes how the mailer script is shelled out in Ruby 1.8.7; now the
  environment variables are properly formatted and standard error is
  piped to standard out

Exposes the regression issue in #1386

The `preview_path` feature of `ActionMailer` was added in Rails 4.1.
Versions before that release are not able to handle that configuration.
The regression issue occurred in part to the spec suite overlooking
changes to the environment and how that affects the shelling out.

Additionally, the previous changes noted that the configuration for a
particular module always responds to a setting; even when it should not.
However, while this logic was noted for `show_previews`, a few lines
further down, that same logic was overlooked for `preview_path`.

It appears the only available methods for detecting `ActionMailer`, and
it's features, is tasting the top-level `config` in addition to checking
the Rails version string.

[ci skip]

Back port pull request #1388 from rspec/fix-regression-mailer-preview-path