Really fix support for :content-type in define-module.

After this fix, specifying content-type in define-module actually works.

Bug reported at https://groups.google.com/forum/#!topic/restas/nwwZLniZca8

This patch just calls `find-pkgmodule-traits` in `route.lisp`. However, would you prefer to define `pkgmodule-traits-content-type` in `module.lisp` and call it from `route.lisp` instead?