Allow boolean values for column[i].searchable

When dt Request is sent as application/json instead of multipart/form-data
and when in ColumnDef boolean values are used the search does not work. 
 
(Angular 6 HTTPClient for example)

might affect #1789

Author: sgotre

src/Utilities/Request.php

@@ -154,10 +154,20 @@ public function searchableColumnIndex()
     public function isColumnSearchable($i, $column_search = true)
     {
         if ($column_search) {
-            return $this->request->input("columns.$i.searchable", 'true') === 'true' && $this->columnKeyword($i) != '';
+            return 
+                (
+                    $this->request->input("columns.$i.searchable", 'true') === 'true' 
+                    || 
+                    $this->request->input("columns.$i.searchable", 'true') === true 
+                ) 
+                && $this->columnKeyword($i) != '';
         }
 
-        return $this->request->input("columns.$i.searchable", 'true') === 'true';
+        return ( 
+            $this->request->input("columns.$i.searchable", 'true') === 'true' 
+            || 
+            $this->request->input("columns.$i.searchable", 'true') === true
+        );
     }
 
     /**