Update changes and add explanatory comment

Author: xdg

Changes

@@ -2,6 +2,10 @@ Release notes for HTTP-Tiny
 
 {{$NEXT}}
 
+ [FIXED]
+
+ - No longer deletes the 'headers' key from post_form arguments hashref.
+
 0.080 2021-11-05 08:15:46-04:00 America/New_York
 
  - No changes from 0.079-TRIAL.

lib/HTTP/Tiny.pm

@@ -247,6 +247,8 @@ sub post_form {
  }
 
  return $self->request('POST', $url, {
+ # Any existing 'headers' key in $args will be overridden with a
+ # normalized version below.
  %$args,
  content => $self->www_form_urlencode($data),
  headers => {