Templates not found when template_is_class=true and Stache.template_base_path is relative

[rwd]

When we have:

• a gem containing Mustache templates and a Rails engine
• that gem's Rails engine setting Stache.template_base_path to a relative path, e.g. "app/templates"
• a Rails application including that gem in its bundle

... then in general, everything works: Mustache templates are loaded from the application's app/templates directory if they exist, or if not, then from the gem's. View classes are created in the application's app/views directories.

The problem is when Stache::Mustache::Handler#compile identifies the template as a class, in our case a false positive because the source includes the string "module". It then looks for a template file in the Stache.template_base_path as though it was an absolute path and:

1. finds nothing because Stache.template_base_path is relative
2. does not detect that no file is found and attempts to read it anyway
3. File.read(file) (L64) fails with a TypeError, with the message "no implicit conversion of nil into String"