* removed rails 5.0 check for mailer preview generator

* Add test fot mailer generator

Use `name.underscore.pluralize` instead of file and table name to handle the case where generating request specs for name spaced files

* Truncate method names further in system tests

ref: https://github.com/rspec/rspec-rails/pull/1999/files

If system test fails, a screenshot is taken, but if the file name exceeds 255 characters(e.g. using turnip), the following error will be output.

```
File name too long @ rb_sysopen
```

The file name is prepended with `failures_` so further truncation is needed.

This is a partial backport of #2069 that doesn't include changes on
"have_enqueued_mail" tests.

When expecting no job to be enqueued, the matcher would expect exactly
one job not to have been enqueued. This commit changes so that it now expects
no jobs to have been enqueued.

3.8.2 release