Allow dot syntax when checking if a session has a value

[adamtomat]

Given:

session()->put([
    'foo' => [
        'bar' => 123,
    ],
]);

I expected session()->has('foo.bar') to return true, however it returns false because it currently doesn't support dot syntax.

This MR is to allow dot syntax for the has method on the session store object.

[coveralls]

Coverage remained the same at 88.698% when pulling 7f586de on allow-dot-syntax-for-session-has into d877406 on master.

[joelambert]

@adamtomat we need to add a test for this behaviour before merging.