`SENSITIVE_HEADERS` checks should not be case-sensitive

[Moelf]

[Found by ZeroPath and manually checked by me]

https://github.com/JuliaWeb/HTTP.jl/blob/e7feb99bf1d4165c7e3e28171eefeb7652ec2773/src/clientlayers/RedirectRequest.jl#L44-L45

https://github.com/JuliaWeb/HTTP.jl/blob/e7feb99bf1d4165c7e3e28171eefeb7652ec2773/src/clientlayers/RedirectRequest.jl#L69-L74

if there's a header like authorization, we would misjudge it as being not sensitive and forward it, which is a security flaw