index.js
function getHeaderNames (res) {
  return typeof res.getHeaderNames !== 'function'
    ? Object.keys(res._headers || {})
    : res.getHeaderNames()
}
function getHeaderNames (res) {
  return typeof res.getHeaderNames !== 'function'
    ? Object.keys(res._headers || {})
    : res.getHeaderNames()
}
function getHeaderNames ( res ) { return typeof res . getHeaderNames !== 'function' ? Object . keys ( res . _headers || { } ) : res . getHeaderNames ( ) }
Get the header names on a respnse.

@param {object} res
@returns {array[string]}
@private
Get the header names on a respnse .