invalid usage of delete operator

There is usage of the `delete` operator in the method `add` (browser/queue.js). This construction is not valid in the `strict mode` so it leads to fail when FuncUnit is imported as a ES6 module.

```javascript
add = function(handler, error, context) {
if(handler instanceof Function) {
if(typeof error === 'object') {
context = error;
delete error;
}

error = (error && error.toString()) || 'Custom method has failed.';
```
Found in the npm version 3.7.0