Bootstrap/Load: Correct sending the X-Pingback header in WP::send_headers().

The logic for sending the `X-Pingback` header for single posts that allow pings was recently moved from `WP::handle_404()`​ to a more appropriate place in `WP::send_headers()`.

To check whether pings are open for a particular post, that logic relies on the `$wp_query` global, which is declared in `WP::handle_404()`, but not in `WP::send_headers()`

This commit ensures that `$wp_query` is globalized in `WP::send_headers()` too, so that the check works as expected.

Follow-up to [54250].

Props strategio, sabernhardt, dlh, davidbaumwald, SergeyBiryukov.
Fixes #56840.

git-svn-id: https://develop.svn.wordpress.org/trunk@54636 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/wp/sendHeaders.php

@@ -19,4 +19,19 @@ class Tests_WP_SendHeaders extends WP_UnitTestCase {
 
 		$this->go_to( home_url() );
 	}
+
+	/**
+	 * @ticket 56840
+	 */
+	public function test_send_headers_sets_x_pingback_for_single_posts_that_allow_pings() {
+		add_action(
+			'wp_headers',
+			function ( $headers ) {
+				$this->assertArrayHasKey( 'X-Pingback', $headers );
+			}
+		);
+
+		$post_id = self::factory()->post->create();
+		$this->go_to( get_permalink( $post_id ) );
+	}
 }