Add setVariables method to GroovyServlet for easy extending of binded vars in custom classes

TemplateServlet already contains a setVariables method which can be used by custom classes extending TemplateServlet.
Using this method the bindings can be extended very easily.

To bad the GroovyServlet does not has this feature.

The attached paths solves this.

• method setVariables added
• in service() after creating the binding, a call to setVariables is added