Makes regular expression selection on collections key case insensitive

This issue was initially reported by @michaelgranzow-avi on #2296.

@airween made an initial attempt to provide a fixed at #2107; As a
consequence of the pull request review - provided by @victorhora,
@zimmerle, and @michaelgranzow-avi - @airween made a second attempt
at #2297. After reviewing by @martinhsv, @zimmerle, I have absorbed
the essential pieces from @airween patch into this one.

This patch differs from @airween's because @airween's patches were
partially working: Key exclusions with regex weren't covered, same
for anchored variables (e.g. ARGS). During the review, I have
highlighted the importance of having elementary test cases. A simple
test case on ARGS could spot the issue. Since that is an important
fix, I don't want to hold this for one more review cycle; therefore,
I am committing the fix myself.

Thank you all involved in the solution of this very own issue.

CHANGES

@@ -1,6 +1,9 @@
 v3.x.y - YYYY-MMM-DD (to be released)
 -------------------------------------
 
+  - Regex key selection should not be case-sensitive
+    [Issue #2296, #2107, #2297 - @michaelgranzow-avi, @victorhora,
+                                 @airween, @martinhsv, @zimmerle]
   - Fix: Only delete Multipart tmp files after rules have run
     [Issue #2427 - @martinhsv]
   - Fixed MatchedVar on chained rules