Rewrite OutputPage::addVaryHeader

Rewrite OutputPage::addVaryHeader

Rewrite OutputPage::addVaryHeader which had a very confusing
structure.

There is one breaking change: the $option argument was declared
as array|null, but the function accepted everything and showed
inconsistent behavior; e.g.

    $op->addVaryHeader( 'Foo', 'bar' )

resulted in 'X-Vary-Options: Foo;bar' but

    $op->addVaryHeader( 'Foo' )
    $op->addVaryHeader( 'Foo', 'bar' )

resulted in 'X-Vary-Options: Foo'. With the patch, non-array
arguments (other than null) result in an error.

Change-Id: Id31d95fe27b01b00ec8a1d7a3996275fc0aacf3c