uwu: flexible and accurate headers

[joshxyzhimself]

Problem

• our headers has custom format like x_forwarded_for
• we cannot parse range headers yet
• we cannot use custom headers like headers['X-UPLOAD-ID'] or headers['X-IDEMPOTENCY-KEY']

Fix

• Use standard Headers object
  • https://developer.mozilla.org/en-US/docs/Web/API/Headers
• Use a function that lets the user extract custom headers from requests

[joshxyzhimself]

fix

• export constant default_headers
• used Map<string, string> instead of Headers for consistency in both request and response object; header keys are now case sensitive
• default_headers is global, cannot set custom headers per middleware yet due to chained middleware support