Fix setting text x or y to 0

Because of the use of the construction "@x = x or @x" there was no way
to set x (or y) to 0, since 0 is falsy.  Fix to use separate outer tests
and unconditional assignment.
Merge pull request #181 from bdowning/master

Fix setting text x or y to 0
Merge branch 'master' of https://github.com/devongovett/pdfkit