bugfix when using several keywords

using the same prevents from using several keywords. An additional array is used when there is more than one keyword.
erase $where[$k] in a clean way before setting it

There is no guarantee that keys will always be numeric keys beginning with 0 and 1 so that the former values in $where[$k] would be erased.
Merge pull request #3 from vrcAlbert/patch-2

bugfix when using several keywords