[index] Update and improve parameter handling for bridge and cache

- Use 'array_diff_key' instead of 'unset'
- Remove parameters for caches

By removing certain parameters for caches, the loading times can be
improved considerably:

* action: It doesn't matter which action the user took to generate
feed items.

* format: This has the biggest impact on performance, because cached
items are now shared between different formats (i.e. try switching
between Atom, Html and Mrss and compare previous vs. now). If a
server handles lots of requests, this may even reduce bandwidth if
the same contents are requested for different formats.

* _noproxy: The proxy behavior has no impact on the produced items,
so it can be ignored.

* _cache_timeout: This is another option which might impact performance
for some servers, especially if 'custom_timeout' has been enabled in
the configuration. Requests with different cache timeouts no longer
result in separate cache files.

index.php

@@ -188,24 +188,43 @@ try {
 			$cache_timeout = filter_var($params['_cache_timeout'], FILTER_VALIDATE_INT);
 		}
 
+		// Remove parameters that don't concern bridges
+		$bridge_params = array_diff_key(
+			$params,
+			array_fill_keys(
+				array(
+					'action',
+					'bridge',
+					'format',
+					'_noproxy',
+					'_cache_timeout',
+				), '')
+		);
+
+		// Remove parameters that don't concern caches
+		$cache_params = array_diff_key(
+			$params,
+			array_fill_keys(
+				array(
+					'action',
+					'format',
+					'_noproxy',
+					'_cache_timeout',
+				), '')
+		);
+
 		// Initialize cache
 		$cache = Cache::create('FileCache');
 		$cache->setPath(CACHE_DIR);
 		$cache->purgeCache(86400); // 24 hours
-		$cache->setParameters($params);
-
-		unset($params['action']);
-		unset($params['bridge']);
-		unset($params['format']);
-		unset($params['_noproxy']);
-		unset($params['_cache_timeout']);
+		$cache->setParameters($cache_params);
 
 		// Load cache & data
 		try {
 			$bridge->setCache($cache);
 			$bridge->setCacheTimeout($cache_timeout);
 			$bridge->dieIfNotModified();
-			$bridge->setDatas($params);
+			$bridge->setDatas($bridge_params);
 		} catch(Error $e) {
 			http_response_code($e->getCode());
 			header('Content-Type: text/html');