Handle also singlepart messsages

[benningm]

When a singlepart message is passed to the delivery method
an undefined method error is raised:

undefined method `body' for nil:NilClass>

This is because the message class expects the content to be a mime-part.
In case of a singlepart message the content is in the message body.

This will use the body on singlepart messages and the text_part for
multipart messages.
It also extends the tests to cover the changed code.

[geetotes]

@benningm This looks good to me -- are you running this gem off your own branch? Looks like I've got some test stuff on my end to clean up, so I'd like to do this and merge your change in with that and release a new version

[benningm]

The tests are broken. I fixed the existing tests. In my test runs the remaining error where 'pending' tests.

In delivery_method test i moved the parameters channel and webhook from the initializer to the right place in the mail.

In the message tests i added the .result() call to cover the templating and trigger the singlepart error in the tests.

I'm currently implementing slack messaging in one of my rails apps and wanted to use the action_mailer with your gem against a mattermost server (Slack API compatible).
I'll try use the gem from my modified branch for now.