Coding Standards: Use self:: for static member reference in WP_Http.

This resolves a PHPCS error:
{{{
Squiz.Classes.SelfMemberReference.NotUsed - Must use "self::" for local static member reference
}}}

Follow-up to [20767], [37428], [38164].

Props dilipbheda.
Fixes #63346.

git-svn-id: https://develop.svn.wordpress.org/trunk@60191 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/class-wp-http.php

@@ -329,7 +329,7 @@ public function request( $url, $args = array() ) {
 
 		// WP allows passing in headers as a string, weirdly.
 		if ( ! is_array( $parsed_args['headers'] ) ) {
-			$processed_headers      = WP_Http::processHeaders( $parsed_args['headers'] );
+			$processed_headers      = self::processHeaders( $parsed_args['headers'] );
 			$parsed_args['headers'] = $processed_headers['headers'];
 		}
 
@@ -368,7 +368,7 @@ public function request( $url, $args = array() ) {
 
 		// If we've got cookies, use and convert them to WpOrg\Requests\Cookie.
 		if ( ! empty( $parsed_args['cookies'] ) ) {
-			$options['cookies'] = WP_Http::normalize_cookies( $parsed_args['cookies'] );
+			$options['cookies'] = self::normalize_cookies( $parsed_args['cookies'] );
 		}
 
 		// SSL certificate handling.
@@ -1076,7 +1076,7 @@ public static function handle_redirects( $url, $args, $response ) {
 			$redirect_location = array_pop( $redirect_location );
 		}
 
-		$redirect_location = WP_Http::make_absolute_url( $redirect_location, $url );
+		$redirect_location = self::make_absolute_url( $redirect_location, $url );
 
 		// POST requests should not POST to a redirected location.
 		if ( 'POST' === $args['method'] ) {