feat: support Response.buffer(), Response.json() and Response.text() (#4063)

This patch:
- implements Response.buffer() and other methods
- splits out relevant tests into a separate test suites
- implements `testServer.enableGzip()` method to optionally gzip
  certain routes in tests
- adds tests to make sure `Response.text()` returns expected results
  for binary and compressed responses.

test/navigation.spec.js

@@ -499,7 +499,7 @@ module.exports.addTests = function({testRunner, expect, Errors, CHROME}) {
       const error = await navigationPromise;
       expect(error.message).toBe('Navigating frame was detached');
     });
-    it_fails_ffox('should return matching responses', async({page, server}) => {
+    it('should return matching responses', async({page, server}) => {
       // Disable cache: otherwise, chromium will cache similar requests.
       await page.setCacheEnabled(false);
       await page.goto(server.EMPTY_PAGE);