module Grape::DSL::Headers

4. Delete a specifc header key-value pair
3. Retrieve all headers that have been set
2. Retrieve a specifc header value by key
1. Set a specifc header value by key
This method has four responsibilities:

def header(key = nil, val = nil)
  if key
    val ? header[key.to_s] = val : header.delete(key.to_s)
  else
    @header ||= Grape::Util::Header.new
  end
end