Support Merge Collection/Map using + Operator

[hantsy]

To simplify the operators of concatenating Collections/Maps.

• aList + aSet returns a new list
• aMap+ anotherMap returns a new Map.

[markt-asf]

This needs to be more precisely defined. I'd lean towards referencing an existing method (addAll(Collection) / putAll(Map)) rather than trying to define our own merge function. Cleaner to write the spec, easier for users to understand and easy to implement.

[markt-asf]

I have an implementation of this in the current Tomcat 12 development branch. You can download SNAPSHOT build a standard distribution from:
https://repository.apache.org/content/groups/snapshots/org/apache/tomcat/tomcat/12.0.0-M1-SNAPSHOT/

Embedded and individual JARs are also available.

If you could try that and and confirm whether or not it works for you that would be great. I can then work on the changes required to the specification document and some TCK tests.

[markt-asf]

It has been over a month without feedback. The operator works as expected for me so I am going to proceed with the spec updates and TCK tests for 6.1.