feat(render): improve ergonomics of send_file
feat(render): improve ergonomics of send_file

anvie/nickel.rs,BenKnigge/nickel.rs,ixjf/nickel.rs,jolhoeft/nickel.rs,nikomatsakis/nickel.rs,SimonPersson/nickel.rs,Ye-Yong-Chi/nickel.rs,nickel-org/nickel.rs,Ryman/nickel.rs