fix: test if gzip extension is available (#308)

If "gzip" PHP extension is not loaded and there is no substitute in
place, gzencode() does not exist and generation of compressed files will
fail.
Add a simple function_exists() test to prevent this.

inc/class-cachify-hdd.php

@@ -36,9 +36,12 @@ public static function is_gzip_enabled() {
 		/**
 		 * Filter that allows to enable/disable gzip file creation
 		 *
-		 * @param bool $create_gzip_files Whether to create gzip files. Default is `true`
+		 * @param bool $create_gzip_files Whether to create gzip files. Default is `true`, if gzip is available.
 		 */
-		return apply_filters( 'cachify_create_gzip_files', true );
+		return apply_filters(
+			'cachify_create_gzip_files',
+			function_exists( 'gzencode' )
+		);
 	}
 
 	/**

tests/test-cachify-hdd.php

@@ -33,19 +33,24 @@ public function test_stringify_method() {
 	 * Test GZip availability.
 	 */
 	public function test_is_gzip_enabled() {
-		self::assertTrue( Cachify_HDD::is_gzip_enabled(), 'GZip should be enabled by default' );
+		$gzip_available = function_exists( 'gzencode' );
+		self::assertSame(
+			$gzip_available,
+			Cachify_HDD::is_gzip_enabled(),
+			'GZip should be ' . ( $gzip_available ? 'enabled' : 'disabled' ) . ' by default'
+		);
 
 		$capture = null;
 		add_filter(
 			'cachify_create_gzip_files',
-			function ( $original ) use ( &$capture ) {
+			function ( $original ) use ( &$capture, $gzip_available ) {
 				$capture = $original;
 
-				return false;
+				return ! $gzip_available;
 			}
 		);
-		self::assertFalse( Cachify_HDD::is_gzip_enabled(), 'GZip should be disabled by filter' );
-		self::assertTrue( $capture, 'Filter was not applied' );
+		self::assertSame( ! $gzip_available, Cachify_HDD::is_gzip_enabled(), 'GZip should be changed by filter' );
+		self::assertSame( $gzip_available, $capture, 'Filter was not applied' );
 	}
 
 	/**