WP_Object_Cache incr & decr can use a more strict type checking on offset

[jipmoors]

While working on improving the unit tests for related ticket #28759 I wrote some tests that could not pass on the current code: passing an non-empty array as offset.

Despite of it being an edge case, I want to have my tests pass, so I provide a patch.

[jipmoors]

Type check on offset

[johnbillion]

Which tests/assertions in your patch on #28759 fail currently?

[jipmoors]

Test increment with an array with entries.

wp_cache_set( $key, 2 );
$this->assertEquals( 2, wp_cache_incr( $key, array() ) );

Results in 3 instead of 2.

[jipmoors]

Simplified the code.