Current Behavior

X-Forwarded-Proto header for upstream has been set as a table

Expected Behavior

X-Forwarded-Proto header for upstream should be set as a string

Error Logs

No response

Steps to Reproduce

1. create a simple route:

curl http://127.0.0.1:9080/apisix/admin/routes/1 -H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -i -d '
{
    "uri": "/test",
    "upstream": {
        "type": "roundrobin",
        "nodes": {
            "127.0.0.1:8080": 1
        }
    }
}'

2. visit the route:

curl 127.0.0.1:9080/test -H "X-Forwarded-Proto: http" -H "X-Test: test"  -H "X-Forwarded-Proto: http" -H "X-Test: test"

3. the upstream will receive:

k: User-Agent, v: [curl/7.64.1]
k: X-Forwarded-For, v: [127.0.0.1]
k: X-Forwarded-Port, v: [9080]
k: X-Forwarded-Proto, v: [table: 0x1b70daa8]
k: X-Forwarded-Host, v: [127.0.0.1]
k: Accept, v: [*/*]
k: X-Test, v: [test test]
k: X-Real-Ip, v: [127.0.0.1]

see:
https://github.com/apache/apisix/blob/master/apisix/init.lua#L230-L233

Environment

• APISIX version (run apisix version): master branch
• Operating system (run uname -a): Darwin Kernel Version 19.6.0
• 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):