Allow mixed case Upgrade headers
Allow mixed case Upgrade headers

Caddy expects websocket to be completely lowercase.
Some applications send websocket upgrade headers like the following:
`Upgrade: WebSocket`

This change allows all variations of websocket.
tw4452852/caddy,cez81/caddy,tpng/caddy,yukimochi/caddy,cez81/caddy,evermax/caddy,dusty/caddy,DenBeke/caddy,joshix/caddy,tobya/caddy,evermax/caddy,yukimochi/caddy,pedronasser/caddy,devangels/caddy,pyed/caddy,tw4452852/caddy,DenBeke/caddy,pyed/caddy,joshix/caddy,yukimochi/caddy,dusty/caddy,dusty/caddy,devangels/caddy,cez81/caddy,elcore/caddy,tpng/caddy,captncraig/caddy,captncraig/caddy,tobya/caddy,elcore/caddy,pedronasser/caddy,pedronasser/caddy,ollie314/caddy,evermax/caddy,tobya/caddy,ollie314/caddy,elcore/caddy,pyed/caddy,tw4452852/caddy,ollie314/caddy,tobya/caddy,pyed/caddy,DenBeke/caddy,DenBeke/caddy,devangels/caddy,evermax/caddy,joshix/caddy,tw4452852/caddy,ollie314/caddy,captncraig/caddy,dusty/caddy,tpng/caddy,pedronasser/caddy,tpng/caddy