Update key_server default value and add test
fix typo
should be undef without quote and remove default key_server test
Add default key_server test back
Clarify tests for aptly:key_server

To describe the behaviour we're testing/expecting when not providing a
value. Rename the first `context` block and add descriptions to each of the
`it` blocks.
Clarify puppetdoc for aptly:key_server

To indicate that it's the hostname of a key server (rather than file
location of a key) and what the behaviour is if you don't specify a value.
[Fixes #11] Merge branch 'pr/11'

I've tacked a couple of documentation tweaks to the end of this PR branch.