Fix RegExp coercion

- Do not coerce RegExp literal/objects into a string

lib/sql.js

@@ -712,6 +712,8 @@ SQLConnector.prototype.buildExpression = function(columnName, operator, columnVa
     case 'nlike':
       sqlExp += ' NOT LIKE ';
       break;
+    // this case not needed since each database has its own regex syntax, but
+    // we leave the MySQL syntax here as a placeholder
     case 'regexp':
       sqlExp += ' REGEXP ';
       break;
@@ -797,6 +799,9 @@ SQLConnector.prototype._buildWhere = function(model, where) {
             }
           }
         }
+      } else if (operator === 'regexp' && expression instanceof RegExp) {
+        // do not coerce RegExp based on property definitions
+        columnValue = expression;
       } else {
         columnValue = this.toColumnValue(props[key], expression);
       }