Don't step on a previously-specified user-agent

Signed-off-by: Ben Copsey <ben@allseeing-i.com>

@@ -391,6 +391,7 @@ static NSError *ASITooMuchRedirectionError;
 	
 	// Set a logical user agent (on iPhones, for now)
 #if TARGET_OS_IPHONE
+	if ([[self requestHeaders] objectForKey:@"User-Agent"] == nil) {
 		UIDevice *device = [UIDevice currentDevice];
 		NSBundle *bundle = [NSBundle mainBundle];
 		NSLocale *locale = [NSLocale currentLocale];
@@ -405,6 +406,7 @@ static NSError *ASITooMuchRedirectionError;
 							   ];
 		
 		[self addRequestHeader:@"User-Agent" value:userAgent];
+	}
 #endif
 	
 	// Accept a compressed response