hmac.digest([encoding])
计算使用 hmac.update() 传入的所有数据的 HMAC 摘要。如果提供了 encoding,则返回一个字符串;否则返回 Buffer;
¥Calculates the HMAC digest of all of the data passed using hmac.update().
If encoding is
provided a string is returned; otherwise a Buffer is returned;
Hmac 对象在 hmac.digest() 被调用后不能再次使用。多次调用 hmac.digest() 将导致抛出错误。
¥The Hmac object can not be used again after hmac.digest() has been
called. Multiple calls to hmac.digest() will result in an error being thrown.