Fix internal tokens used in `\prop_get:Nn` (fixes #144)

This fix addresses the immediate issue but also the fact that the behaviour
was incorrect when the key was not found. A small number of tests are also
added.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4468 de43f980-851b-0410-b2f7-c40aca1f87e0
Step version (CTAN snapshot)

As this is an 'emergency' snapshot to fix only one issue in `l3candidates`,
the other files are not altered.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4469 de43f980-851b-0410-b2f7-c40aca1f87e0