Use httphelper and cache response even when it is empty


Fixes #12164
That said, the tests will fail:
```
➜  tests git:(fix-12164) ✗ phpunit lib/updater.php
PHPUnit 4.3.4 by Sebastian Bergmann.

Configuration read from /Users/lreschke/Programming/core/tests/phpunit.xml.dist

........F

Time: 148 ms, Memory: 12.00Mb

There was 1 failure:

1) OC\UpdaterTest::testCheck
Failed asserting that an array contains 'version'.

/Users/lreschke/Programming/core/tests/lib/updater.php:50

FAILURES!
Tests: 9, Assertions: 9, Failures: 1.
```