GRAILS-9775 - Improve bindable handling for relationships

This problem may related to the 071ca8f8c9f6ebc92 commit as part of GRAILS-8972.
Merge branch '2.0.x' into 2.1.x
Merge branch '2.1.x' into 2.2.x

Conflicts:
	grails-web/src/main/groovy/org/codehaus/groovy/grails/web/binding/DefaultASTDatabindingHelper.java
	grails-web/src/test/groovy/org/codehaus/groovy/grails/web/binding/DefaultASTDatabindingHelperSpec.groovy