Improve test for layout on go_to_page
Restore margins on go_to_page()

We were not copying page margins when creating new pages, just
referencing the same hash used by previous pages. We should
really redesign this code to be mostly immutable, but this
fix will hopefully solve #272 for now.
Merge pull request #794 from prawnpdf/margin-bug

Properly restore margins on go_to_page