Links inside a stamped text are not clickable

[hidakatsuya]

My environments are:

• MacOSX 10.10
• Prawn 0.12.0, 1.3.0
• Ruby 1.9.3, 2.1.3

Prawn::Document.generate 'foo.pdf' do
  create_stamp 'bar' do
    text '<b>Prawn</b> <link href="http://github.com">GitHub</link>', inline_format: true
  end
  stamp 'bar'
end

Formats except link are properly rendered, but links are not clickable.

[practicingruby]

I believe this is a duplicate of #498 but actually gets us lower down the stack. Leaving open until we confirm they're related.