Fixed crash under non-ARC

CGDWebServer/GCDWebServer.m

@@ -91,9 +91,9 @@ void GCDLogMessage(long level, NSString* format, ...) {
 NSString* GCDWebServerGetMimeTypeForExtension(NSString* extension) {
   static NSDictionary* _overrides = nil;
   if (_overrides == nil) {
-    _overrides = @{
-                   @"css": @"text/css"
-                   };
+    _overrides = [[NSDictionary alloc] initWithObjectsAndKeys:
+                  @"text/css", @"css",
+                  nil];
   }
   NSString* mimeType = nil;
   extension = [extension lowercaseString];