Fix toString override problem of GString

In some cases we can't override toString() method of GString

When we coerce GString to String method 
org.codehaus.groovy.reflection.stdclasses.StringCachedClass.coerceArgument(Object)
invoked.
In this method toString() method of GString invokes directly, so if we override toString method of GString in ExtensionModule it does not work.