[FIX] Email Body retrieval logic in IMAP Listener

[ayeshLK]

Purpose

$subject

Fixes #2777

Examples

N/A

Checklist

• Linked to an issue
• Updated the changelog
• Added tests
• Updated the spec

[codecov]

Codecov Report

Merging #468 (e7e3298) into 2201.0.x (bfaf797) will decrease coverage by 0.08%.
The diff coverage is 76.47%.

@@              Coverage Diff               @@
##             2201.0.x     #468      +/-   ##
==============================================
- Coverage       80.97%   80.88%   -0.09%     
- Complexity        159      160       +1     
==============================================
  Files              22       22              
  Lines            1051     1062      +11     
  Branches          205      207       +2     
==============================================
+ Hits              851      859       +8     
- Misses            115      117       +2     
- Partials           85       86       +1     

Impacted Files	Coverage Δ
...o/ballerina/stdlib/email/util/EmailAccessUtil.java	77.89% <76.47%> (-0.22%)	⬇️

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[shafreenAnfar]

@ayeshLK and I had a chat and decided to update the testcase names. The fix is simply about making MIME type case insensitive. The testcases are simply there to validate this.