Add testcase for get-method overload

[SjonHortensius]

Test change introduce in #209 to fix #126

[sodabrew]

This presently fails with:

Warning: Declaration of test_overload_get::get($key, $cache_cb = NULL, &$cas_token = NULL)
should be compatible with Memcached::get($key, $cache_cb = NULL, $get_flags = NULL)
in tests/overload_get.php

[SjonHortensius]

I originally added this test to make sure php7 userland code will be able to overload Memcached::get successfully. After adding this test, dc5b22a fixed the issue, but broke this test.

I've added a commit that fixes the test by implementing the new get() signature

[sodabrew]

Thanks for your responsiveness @SjonHortensius! I'm going to close this PR without merging, however. The test at this point is no longer checking the underlying (and unresolved) PHP
https://bugs.php.net/bug.php?id=66331 and so the effect is just to test normal class inheritance, which is basic PHP and not something we need tests for here.

The php-memcached API going forward adds a flag field to get and getMulti, with the value Memcached::GET_EXTENDED changing the methods to return an array. Because the flags field is a pass-by-value integer, it's possible to subclass or override the method using the normal PHP syntax without running into the corner case with the references.

[SjonHortensius]

@sodabrew thanks for explaining. I agree with your statement, and I hope php-memcached will only implement signatures in the future which can be overloaded normally.