Fix E_STRICT error on Index & SearchIndex

E_STRICT: Declaration of
vierbergenlars\Norch\ODM\SearchIndex::addDocument() should be compatible
with vierbergenlars\Norch\SearchIndex\Index::addDocument(array
$document) in
/home/lars/Documents/GitHub/Norch-PHP-Client/lib/vierbergenlars/Norch/ODM/SearchIndex.php
on line 42

Checks for type are now done inline.