Fix morph relation support

Recent PR #2580 fails when a multiple level relationship is used ($relation string has several dot separated relations). I've created this fix in order to check only the first relation (I don't really know how Laravel behaves with multi level relationships when all or any are morphed).

Author: PabloCamp0s

src/EloquentDataTable.php

@@ -92,7 +92,7 @@ protected function compileQuerySearch($query, $columnName, $keyword, $boolean =
             return parent::compileQuerySearch($query, $columnName, $keyword, $boolean);
         }
 
-        if ($this->query->getModel()->$relation() instanceof MorphTo) {
+        if ($this->isMorphRelation($relation)) {
             $query->{$boolean . 'WhereHasMorph'}($relation, '*', function (Builder $query) use ($column, $keyword) {
                 parent::compileQuerySearch($query, $column, $keyword, '');
             });
@@ -122,6 +122,25 @@ protected function resolveRelationColumn($column)
         return $this->joinEagerLoadedColumn($relation, $columnName);
     }
 
+    /**
+     * Check if a relation is a morphed one or not.
+     *
+     * @param  string $relation
+     * @return bool
+     */
+    protected function isMorphRelation($relation)
+    {
+        $isMorph = false;
+        if ($relation !== null && $relation !== '')
+        {
+            $relationParts = explode('.', $relation);
+            $firstRelation = array_shift($relationParts);
+            $isMorph       = $this->query->getModel()->$firstRelation() instanceof MorphTo;
+        }
+
+        return $isMorph;
+    }
+
     /**
      * Check if a relation was not used on eager loading.
      *