[NSURL URLWithString:] 返回nil

具体问题原因是url中输入的有中文，那么这个就看作非法的字符无法识别。这种的必须使用post方式来发送消息。具体为：

tmp = mainurl;
[parameters appendString:key];
[parameters appendString:value];

NSURL * downloadUrl = [NSURL URLWithString:self.strURL];
NSMutableURLRequest *requestL = [NSMutableURLRequest requestWithURL:downloadUrl
cachePolicy:NSURLRequestReloadIgnoringCacheData
timeoutInterval:60.0];
[requestL setHTTPMethod:@"POST"];
NSString *postString = parameters;
[requestL setHTTPBody:[postString dataUsingEncoding:NSUTF8StringEncoding]];
NSLog(@"the post body is %@",[NSString stringWithFormat:@"%@", postString]);