Use prefix at joinWhere

[DanielBGomez]

I was trying the joinWhere method [#294] but it wasn't working as expected [#513] so i took a look at the code and realized that the table prefix isn't used [#513]. This affects at _buildJoin() because in the condition if (!empty($this->_joinAnd) && isset($this->_joinAnd[$joinStr])) { the $joinStr has the prefix.