a ClassCastException is thrown when EL is used in conjunction with a custom PageContext wrapper

The following method in org.apache.jasper.runtime.PageContextImpl

```
private static ExpressionFactory getExpressionFactory(
PageContext pageContext) {

PageContextImpl pc =
(PageContextImpl) JspContextWrapper.getRootPageContext(pageContext);
return pc.getJspApplicationContext().getExpressionFactory();
}
```

results in the exception.

However, it's possible to accomplish the exact same thing in a compliant way, without a cast, using:

```
JspApplicationContext jspApplicationContext =
JspFactory.getDefaultFactory().getJspApplicationContext(
pageContext.getServletContext());

return jspApplicationContext.getExpressionFactory();
```
#### Environment
any
#### Affected Versions
[current]