various improvements for Server#widget_banner_url
various improvements for Server#widget_banner_url

VxJasonxV/discordrb,VxJasonxV/discordrb,meew0/discordrb,Roughsketch/discordrb,megumisonoda/discordrb,meew0/discordrb,Roughsketch/discordrb,megumisonoda/discordrb