fix: jwt-auth, token exp cannot be set normally #11650

[fanyu-byte]

Description

Swapped the two parameters of core.table.merge in the get_real_payload method, which caused us to be unable to set the expiration time of the token

Fixes #11650

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[fanyu-byte]

when we generate a token, exp on payload not effective

after this change it works:

Please cherry pick this change to another branch

[moonming]

closed, reason is here #11650 (comment)