NSMutableDictionary有两个set方法：
方法一：
- (void)setValue:(nullableObjectType)value forKey:(NSString*)key;
/* Send -setObject:forKey: to the receiver, unless the value is nil, in which case send -removeObjectForKey:.*/
这里的value如果是nil，就相当于移除对应的键值；
- (void)setObject:(ObjectType)anObject forKey:(KeyType)aKey;
anObject
The value for aKey. A strong reference to the object is maintained by the dictionary.
key对应的值，字典会对他进行一次强引用
Important
Raises an NSInvalidArgumentException if anObject is nil. If you need to represent a nil value in the dictionary, use NSNull.
如果anObject为nil，会触发异常。如果你需要代表空，可以用NSNull对象来代替；
aKey
The key for value. The key is copied (using copyWithZone:; keys must conform to the NSCopying protocol). If aKey already exists in the dictionary, anObject takes its place.
value对应的值，如果有旧值，就会把旧值给替换掉；
二者的联系是：
- (void)setValue:(nullableObjectType)value forKey:(NSString*)key;
相当于：
- (void)setObject:(ObjectType)anObject forKey:(KeyType)aKey;
添加nil的时候做了一层保护；因此，我们在使用的时候最好优先使用setValue，这样更加安全；
