Custom ValueConverter does not bind to hasMany collection properly

[joemccall86]

Task List

• Steps to reproduce provided
• Stacktrace (if present) provided
• Example that reproduces the problem uploaded to Github
• Full description of the issue provided (see below)

Steps to Reproduce

1. Register a custom ValueConverter in resources.groovy
2. Include hasMany = [attachments: Attachment] in a domain class.
3. Attempt to bind a value using data binding to collections convention (e.g., attachments[0]=<foo>)

Expected Behaviour

The collection is created utilizing the custom converter.

Actual Behaviour

The collection is not bound.

Environment Information

• Operating System: Fedora 29 Workstation
• Grails Version: 3.2.13, 3.3.9
• JDK Version: openjdk version "1.8.0_191"
• Container Version (If Applicable): N/A

Example Application

https://github.com/joemccall86/data-converter-binding

The master branch is Grails 3.2.13, and the grails_339 branch is Grails 3.3.9. Integration test is included (using the Micronaut HTTP client, and a picture of my cat!)

[graemerocher]

Thanks for the PR