Add additional specs for messages with attachments.
Move deprecation warning in Mail::Message#postmark_attachments to avoid an unneeded noise.
Fix a possible problem in Regexp used in Postmark::Inflector.
Add a spec for Postmark::Inflector.
Remove the hack used to retrieve Postmark attachments from Mail::Message's header.
Add a spec for Postmark::Json.
Add a spec for Postmark::Bounce.
Remove legacy Bounce spec.
Copy the structure of lib directory in spec/unit.
Use spec/data/empty.gif where possible.