[api] use object instead of map for headers (#567)

This patch:

- switches to objects instead of maps for headers (in Request, Response and
page.setExtraHTTPHeaders)
- converts all header names to lower case

Fixes #547, fixes #509

lib/helper.js

@@ -218,9 +218,4 @@ class Helper {
   }
 }
 
-// Polyfill from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries
-if (!Object.entries)
-  Object.entries = obj => Object.keys(obj).map(key => [key, obj[key]]);
-
-
 module.exports = Helper;