Eliminate prepended whitespace when neither prepend configurations are set to true

A bug was introduced where a single white space was being added to a subject line when neither of the prepend configurations were enabled.

This commit refactors the `prepend_custom_subject` method to add a space only when there are strings to prepend. It also introduces a new convenience method `custom_subject` which does the array join
previously done in `prepend_custom_subject`. 

It also reduces the complexity of the method itself, which was introduced before to better handle messages with a `nil` subject.
I'll add 2.1.5 and leave 2.1.0, since that's the latest release.