message: refactor(header): only import langtag macro for tests
diff: #[macro_use] extern crate futures;
  extern crate futures_cpupool;
  extern crate httparse;
  -#[macro_use] extern crate language_tags;
  +#[cfg_attr(test, macro_use)] extern crate language_tags;
  #[macro_use] extern crate log;
  #[macro_use] pub extern crate mime;
  extern crate relay;
  