GRAILS-11673 - improve unit testability of the forward method

This is a breaking change for existing tests which expect the leading "/grails" and trailing ".dispatch" to be there but this change makes the system make more sense.  It is only a breaking change for unit tests and should not affect any running applications.  We will note the change in the release notes.
fix typo in a comment
Merge branch '2.4.x'

Conflicts:
	grails-test-suite-web/src/test/groovy/org/codehaus/groovy/grails/web/mapping/filter/RestfulMappingsFilterTests.groovy
	grails-test-suite-web/src/test/groovy/org/codehaus/groovy/grails/web/mapping/filter/UrlMappingsFilterTests.groovy