[PATCH] Values in PDF Number Trees must be indirect references

See PDF Reference, Third Edition, Section 3.8.5, "Number Trees": the Nums entry
must be an array of key-value pairs where value is an indirect reference to the
object associated with the key.

However, the o.a.f.pdf.PDFNumsArray class does nothing to ensure that elements
added to the array are actually indirectly referenced. So generated PDFs are
strictly speaking invalid.