[Fix] `utils`: encode `+` as `%2B` in iso-8859-1 mode so values round-trip #565

With charset: 'iso-8859-1', a literal + in a key or value is emitted unencoded, so it round-trips back as a space:

qs.stringify({ a: 'b+c' }, { charset: 'iso-8859-1' });

// 'a=b+c'

qs.parse('a=b+c', { charset: 'iso-8859-1' });

// { a: 'b c' }  — the + became a space

The iso-8859-1 path encodes via escape, which leaves + untouched, but a + in a query string decodes to a space, so the value is silently corrupted. The utf-8 path already percent-encodes it (a=b%2Bc). I encode + as %2B in the iso-8859-1 branch too, matching utf-8, so values containing a + survive a round-trip.