Request header key case inconsistencies

[johnjamesjacoby]

RFCs ​7230 and ​7540 define header keys as case-insensitive.

WordPress is consistently inconsistent when it comes to :

• Retrieving headers – always lowercase:

  wp_remote_retrieve_header( $response, 'content-md5' )`
  

• Setting headers – almost always Capital-Case:

  header( 'Content-Type: text/html; charset=utf-8' );
  

• Sitemaps code (from 5.5) uses Title-case:

  header( 'Content-type: application/xml; charset=UTF-8' );
  

(Thankfully, when it comes to actually using the wp_remote_retrieve_header() function (and subsequently the Requests API and WP4.6) it resolves down to Requests_Utility_CaseInsensitiveDictionary so it is all working as defined in the specification.)

Possible code/docs improvements here include:

• standardizing when core uses which case
• explicitly stating that header key parameters are case-insensitive

Related to: #51736, #38231.

[hellofromTonya]

5.9 is in feature freeze and Beta 1 release is < 4 hours away, moving this to 6.0.