Purpose

Replace the header class for something that follows the spec a little more correctly

Changes

getAll() & raw() and all of the URLSearchParams methods are removed

Additional information

• ideally i would like to remove some more own test in node-fetch as the fetch-headers package has it's own test that are run against WPT - but i tough of keeping them for now just to make sure things still works.
• The new Headers implementation should be reverted #1119 (comment)
• also... this will sort of revert fix: Convert Content-Encoding to lowercase #672
  (content-encoding should be case case insensitive and not normalized - so this is now being done in the fetch method now instead)

• I updated & removed some unit test(s)

• fix The new Headers implementation should be reverted #1119