GRAILS-9895 - improve binding String to collection

When binding a String to a collection of some non String type, the conversion was not happening.

Relevant functional tests:
https://github.com/grails/grails-functional-tests/commit/ea1dc74a1b5d32d41761404de40bc0dd1f993691
Merge branch '2.1.x' into 2.2.x