Request::overrideGlobals() may call invalid ini value

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | perhaps
| Deprecations? | no
| Tests pass?   | -
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

According to http://php.net/manual/ja/ini.core.php ,
there's not variable_order, but variables_order (with trailing "s").

Perhaps it breaks BC for some developer who unsets (sets falsy value to )
'request_order' ini value and sets 'variable_order' manually?