Quote all String-based args in gwt:test's gwt.args

This is to ensure it works when paths contains spaces.
Merge branch 'master' of https://github.com/gwt-maven-plugin/gwt-maven-plugin

# By Thomas Broyer
# Via Thomas Broyer
* 'master' of https://github.com/gwt-maven-plugin/gwt-maven-plugin:
  Quote all String-based args in gwt:test's gwt.args