fn header_field<'a, T>(input: T) -> IResult<T, (&'a str, &'a str), u32>
where
  T: 'a,
  T: Slice<Range<usize>> + Slice<RangeFrom<usize>> + Slice<RangeTo<usize>>,
  T: Clone + Offset,
  T: AsBytes + InputIter + AtEof,
  T: PartialEq,
  T: FindSubstring<&'a [u8]>,
  <T as InputIter>::Item: AsChar,
  T: InputTakeAtPosition,
  <T as InputTakeAtPosition>::Item: AsChar
{
    do_parse!(input,
        key: map_res!(
                map!(take_until_and_consume!(b":" as &[u8]),
                     |a| a.as_bytes()),
                std::str::from_utf8) >>
        val: map_res!(
                map!(take_until_and_consume!(b"\r" as &[u8]),
                     |a| a.as_bytes()),
                std::str::from_utf8) >>
        (key.trim(), val)
    )
}