Send custom headers with UIWebView loadRequest

I found that this was the way to add headers to my UIWebView request - override this delegate method:

- (BOOL) webView:(UIWebView*)webView shouldStartLoadWithRequest:(NSURLRequest*)request navigationType:(UIWebViewNavigationType) navigationType

With this code:

BOOL headerIsPresent = [[request allHTTPHeaderFields] objectForKey:@"my custom header"]!=nil;

if(headerIsPresent) return YES;

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    dispatch_async(dispatch_get_main_queue(), ^{
        NSURL *url = [request URL];
        NSMutableURLRequest* request = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:60.0];

        // set the new headers
        for(NSString *key in [self.customHeaders allKeys]){
            [request addValue:[self.customHeaders objectForKey:key] forHTTPHeaderField:key];
        }

        // reload the request
        [self loadRequest:request];
    });
});
return NO;

The answer by Thomas will not work for (most of the) webpages with multiple iFrames. The solution will load an iFrame request over the full UIWebView. E.g. in case it calls loadRequest for a Google advt. (which is in some small iFrame) the advt. is loaded all over the UIWebView & nothing else.