[CMSP-1033] Allow skipping cache control (#40)

* add a filter to skip cache control in the mu-plugin

* adds docs for skip cache control filter

README.md

@@ -80,6 +80,16 @@ Allows you to disable the ability to clear the entire cache from the WordPress a
 add_filter( 'pantheon_cache_allow_clear_all', '__return_false' );
 ```
 
+#### `pantheon_skip_cache_control`
+Allows you to disable the cache control headers that are sent by the Pantheon Page Cache plugin.
+
+**Default Value:** `false`
+
+**Example:**
+```php
+add_filter( 'pantheon_skip_cache_control', '__return_true' );
+```
+
 ### Actions
 #### `pantheon_cache_settings_page_top`
 Runs at the top of the Pantheon Page Cache settings page.

inc/pantheon-page-cache.php

@@ -380,6 +380,19 @@ private function get_cache_control_header_value() {
 	 * @return void
 	 */
 	public function cache_add_headers() {
+		/**
+		 * Filter to skip the cache control header.
+		 *
+		 * @param bool $skip_cache_control Whether to skip the cache control header.
+		 * @see https://github.com/pantheon-systems/pantheon-mu-plugin/issues/37
+		 * @return bool
+		 */
+		$skip_cache_control = apply_filters( 'pantheon_skip_cache_control', false );
+
+		if ( $skip_cache_control ) {
+			return;
+		}
+
 		header( sprintf( 'cache-control: %s', $this->get_cache_control_header_value() ) );
 	}