Request::overrideGlobals() may call invalid ini value

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
'request_order' ini value and sets 'variable_order' manually?
bug #9499 Request::overrideGlobals() may call invalid ini value (denkiryokuhatsuden)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #9499).

Discussion
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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?

Commits
-------

7f43fb8 Request::overrideGlobals() may call invalid ini value