bug: hmac-auth plugin will cause error when use duplicate signature header

[ronething]

Current Behavior

when use duplicate signature header, hmac auth plugin will panic and return 500

Expected Behavior

In APISIX, most of the obtained headers are used directly as strings. Only a few scenarios may be used as arrays, which need to be checked to ensure that the data is used as expected to avoid errors.

If it's a bug, please assign this issue to me, we can change core.request.header function return string instead of table to avoid this problem.

Error Logs

Steps to Reproduce

1、run apisix with master branch
2、add duplicate signature header in TEST 16: verify: invalid signature to file t/plugin/hmac-auth.t

--- more_headers
X-HMAC-SIGNATURE: asdf
X-HMAC-SIGNATURE: asdf

3、run test case for t/plugin/hmac-auth.t, and you can see 500 Internal Server Error

Environment

• APISIX version (run apisix version):
• Operating system (run uname -a):
• OpenResty / Nginx version (run openresty -V or nginx -V):
• etcd version, if relevant (run curl http://127.0.0.1:9090/v1/server_info):
• APISIX Dashboard version, if relevant:
• Plugin runner version, for issues related to plugin runners:
• LuaRocks version, for installation issues (run luarocks --version):

[starsz]

It's a bug, assigned to you to fix it.