Removing AsciiString dep and one unsafe

This library is not about enforcing standards, so the internal use of
AsciiString for headers and status lines is not necessary.

Cargo.toml

@@ -17,7 +17,6 @@ charset = ["encoding"]
 tls = ["rustls", "webpki", "webpki-roots"]
 
 [dependencies]
-ascii = "1"
 base64 = "0.10"
 chunked_transfer = "1"
 cookie = { version = "0.12", features = ["percent-encode"] }