proper signature validation (#13523)

proper signature validation (#13523)

$header_signature could be a typed float (start with 0e and then only numbers) and a float does equal a string when comparing with typed juggle.
eg: 0e123 != "abc" does return false, but 0e123 !== "abc" returns true.

you previously could circumvent the signature check when providing a header signature in the float format (0e...)

Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>