docs: Improved hmac-auth plugin signature string generation instructions

[caixiaomao]

Improved or typo fixed.

When a signature string is generated, an empty string must be wrapped, even if there is no signature information. Otherwise, authentication fails. This needs to be addressed when generating the signature string

Solution

Add the corresponding description to the sample document that generates the sample signature string

[tzssangglass]

hi @caixiaomao , Can you paste the link or picture to the error?

[caixiaomao]

@tzssangglass
It's not a bug, it's documentation that needs fixing。
In the Signature string splicing example
The signing_string generated according to the signature generation formula is:

"GET
/index.html
age=36&name=james
user-key
Tue, 19 Jan 2021 11:33:20 GMT
User-Agent:curl/7.29.0
x-custom-a:test
"

If there is no date or canonical_query_string, The string that needs to be encrypted should look like this, and the documentation should complement this

"GET
/index.html

user-key

User-Agent:curl/7.29.0
x-custom-a:test
"