Default options for `stylesheet_path` and `use_stylesheet` don't work correctly with mailer classes placed in modules.

[ssemakov]

Hi,
Thanks for your work on this gem I found it useful.

Here is one issue with it though. In a case when mailer placed inside of a module default values for stylesheets and stylesheet_path will not be defined correctly.
For instance, if I have
app/mailers/foo/bar_mailer.rb

class Foo::BarMailer < ApplicationMailer
  include InlineStylesMailer
  ...

default value for @stylesheet will be defined as _foo/bar_mailer* and default value for @stylesheet_path will be app/assets/stylesheets. While it seems like it should define default stylesheet name as _bar_mailer* and look for it in app/assets/stylesheets/foo.

I can workaround of this issue by explicitly using stylesheet_path and use_stylesheet, but I think it would be nice to have default options work as expected.

I can submit a PR with a fix if you will be willing to accept it.

In any case thanks for your work on this gem!
-Simon

[billhorsman]

That sounds very reasonable to me, Simon. Your PR would be very welcome!