Wget only recognizes http_proxy, not https_proxy

My Ubuntu 16.04 LTS works behind proxy. I have set HTTP_PROXY and HTTPS_PROXY environmental variables:

HTTP_PROXY=http://web-proxy.corp.xxxxxx.com:8080/
HTTPS_PROXY=https://web-proxy.corp.xxxxx.com:8080/

But wget can’t work:

# wget -P /tmp https://github.com/NVIDIA/nvidia-docker/releases/download/v1.0.0-rc.3/nvidia-docker_1.0.0.rc.3-1_amd64.deb
--2016-07-14 22:51:12--  https://github.com/NVIDIA/nvidia-docker/releases/download/v1.0.0-rc.3/nvidia-docker_1.0.0.rc.3-1_amd64.deb
Resolving github.com (github.com)... 192.30.253.112
Connecting to github.com (github.com)|192.30.253.112|:443... connected.
ERROR: cannot verify github.com's certificate, issued by ‘O=Fortinet Ltd.,CN=FG3K6C3A15800021’:
  Self-signed certificate encountered.
    ERROR: certificate common name ‘FG3K6C3A15800021’ doesn't match requested host name ‘github.com’.
To connect to github.com insecurely, use `--no-check-certificate'.
root@ubuntu:~# wget --no-check-certificate -P /tmp https://github.com/NVIDIA/nvidia-docker/releases/download/v1.0.0-rc.3/nvidia-docker_1.0.0.rc.3-1_amd64.deb

After setting http_proxy and https_proxy:

http_proxy=http://web-proxy.corp.xxxxxx.com:8080/
https_proxy=https://web-proxy.corp.xxxxx.com:8080/
HTTP_PROXY=http://web-proxy.corp.xxxxxx.com:8080/
HTTPS_PROXY=https://web-proxy.corp.xxxxx.com:8080/

Now wget works:

# wget -P /tmp https://github.com/NVIDIA/nvidia-docker/releases/download/v1.0.0-rc.3/nvidia-docker_1.0.0.rc.3-1_amd64.deb     --2016-07-14 22:57:30--  https://github.com/NVIDIA/nvidia-docker/releases/download/v1.0.0-rc.3/nvidia-docker_1.0.0.rc.3-1_amd64.deb
Resolving web-proxy.xxxxxx.hp.com (web-proxy.xxxxxx.hp.com)... xxx.xxx.xxx.xxx
Connecting to web-proxy.xxxxxx.hp.com (web-proxy.xxxxxx.hp.com)|xxx.xxx.xxx.xxx|:8080... connected.
Proxy request sent, awaiting response... 302 Found
......

So we can conclude that wget is picky about uppercase and lowercase words.