Fix bad field name in request HeaderCollection.
Fix bad field name in request HeaderCollection.

--HG--
extra : rebase_source : 6805fa64348bdaa8051c83672f3b30163d5edcb5

servo/rust-http,servo/rust-http,chris-morgan/rust-http,servo/rust-http,Shraddha512/rust-http,Shraddha512/rust-http,servo/rust-http,Shraddha512/rust-http,chris-morgan/rust-http,chris-morgan/rust-http,Shraddha512/rust-http,chris-morgan/rust-http,Shraddha512/rust-http