[#4996] Correct order of merging

- Added test showing issue with merging
- Fixed merging logic to merge new input with old
[#4996] Additional test showing intended use case for input merging

- Provided by @Bakura10
[#4996] Set fieldset as base fieldset

- to ensure input filter does not need to look for fieldset name
[#4996] Fix input retrieval

- It's nested.
Merge branch 'hotfix/4996-alt-test' into hotfix/4996

Merge in tests from @Bakura10
[#4996] Ensure input filter specification is merged last

- to preserver order of operations
[#4996] CS fixes

- trailing whitespace
Merge branch 'hotfix/4996'

Close #4996