fix: remove use of the deprecated OutgoingMessage.prototype._headers

Removes deprecated code. Cannot replace with setHeaders since headers already sent.
In case you still want to obscure the headers from the request-object i suggest you strip down the request object to only include desired attributes, rather than changing the original object.

Co-authored-by: Marco Link <marco.link@contentful.com>
Co-authored-by: Makinwa Olubukola <bukola.makinwa@contentful.com>

lib/contentful.js

@@ -103,10 +103,6 @@ function obscureAuthTokenInResponse (http) {
 
  error.response.config.headers.Authorization = error.response.config.headers.Authorization.replace(token, `Bearer...${token.substr(-5)}`)
 
- if (error.response.request._headers && error.response.request._headers.authorization) {
- error.response.request._headers.authorization = error.response.request._headers.authorization.replace(token, `Bearer...${token.substr(-5)}`)
- }
-
  if (error.response.request._header) {
  error.response.request._header = error.response.request._header.replace(token, `Bearer...${token.substr(-5)}`)
  }