使用ios的UIWebview会自动进行缓存，我们在开发的时候要记得清除Cookie和缓存。
_webView = nil;
[self cleanCacheAndCookie];
调用的方法代码如下：
/**清除缓存和cookie*/
- (void)cleanCacheAndCookie{
//清除cookies
NSHTTPCookie *cookie;
NSHTTPCookieStorage *storage = [NSHTTPCookieStorage sharedHTTPCookieStorage];
for (cookie in [storage cookies]){
[storage deleteCookie:cookie];
}
//清除UIWebView的缓存
NSURLCache * cache = [NSURLCache sharedURLCache];
[cache removeAllCachedResponses];
[cache setDiskCapacity:0];
[cache setMemoryCapacity:0];
}