refactor(headers): Use header!() macro for ETag header

Change `Etag` (only the `E` is capitalized) to `ETag` as it is written
in the relevant RFC.

BREAKING CHANGE: `Etag` header field is now `ETag` header field

src/header/common/mod.rs

@@ -20,7 +20,7 @@ pub use self::content_encoding::ContentEncoding;
 pub use self::content_type::ContentType;
 pub use self::cookie::Cookie;
 pub use self::date::Date;
-pub use self::etag::Etag;
+pub use self::etag::ETag;
 pub use self::expect::Expect;
 pub use self::expires::Expires;
 pub use self::host::Host;