Issue description

Can the port be set for http_to_https in the redirect plugin?

The default https port of apisix is 9443, but after I set http_to_https to true,
visiting "http://test.com:9080/service/"
will redirect to "https://test.com/service/",
my expected to jump to "https://test.com:9443/service/"

Environment

• apisix version (cmd: apisix version):2.6
• OS (cmd: uname -a):Linux xxxxx 4.19.90-2012.5.0.0054.oe1.x86_64 #1 SMP Tue Dec 22 15:58:47 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
• OpenResty / Nginx version (cmd: nginx -V or openresty -V):

nginx version: openresty/1.19.3.1
built by gcc 8.3.1 20190311 (Red Hat 8.3.1-3) (GCC)
built with OpenSSL 1.1.1h  22 Sep 2020 (running with OpenSSL 1.1.1k  25 Mar 2021)

• etcd version, if have (cmd: run curl http://127.0.0.1:9090/v1/server_info to get the info from server-info API):

{"hostname":"xxxxxx","boot_time":1623142227,"etcd_version":"3.4.0","up_time":93656,"last_report_time":1623235829,"id":"xxxxx","version":"2.6"}

• apisix-dashboard version, if have:
• luarocks version, if the issue is about installation (cmd: luarocks --version):/usr/local/bin/luarocks 3.4.0