放在项目中的plist的不能写入。需要放到沙盒中操作。
一，读取：
NSMutableDictionary *dataDic = [NSMutableDictionary dictionaryWithContentsOfFile:self.plistPath];
二、写入：
NSMutableDictionary *dataDic = [NSMutableDictionary dictionaryWithContentsOfFile:self.plistPath];
[dataDic setObject:@""forKey:@"password"];
[dataDic writeToFile:self.plistPath atomically:YES];
注意：写入对象不能不为nil
