Allows to specify a body with custom Content-Type.

[guv]

With Postal 2.0.3 it is not possible to send emails with HTML content instead of plain text.
This adds support for specifying a map as body that contains :content and :type.

For the HTML use case, the body can be specified as:

:body {:type "text/html", :content "<h1>HTML Email</h1>"}