title: How to initial a NSURLRequest with user agent
body: <p>Usually the http user-agent is like:</p>
  
  <pre><code>Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_0 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7A341
  </code></pre>
  
  <p>If I </p>
  
  <pre><code>NSMutableURLRequest *newUserAgentRequest = (NSMutableURLRequest*)[NSURLRequest requestWithURL:self.url];
      NSString *userAgent = [newUserAgentRequest valueForHTTPHeaderField:@"User-Agent"];
  </code></pre>
  
  <p>the userAgent is nil or empty means </p>
  
  <pre><code>NSMutableURLRequest *newUserAgentRequest = (NSMutableURLRequest*)[NSURLRequest requestWithURL:self.url];
  </code></pre>
  
  <p>no user-agent includes, how to initial a request with user agent?</p>