Fix github #537 : force text rendering mode for templates

Add method forget_text_rendering_mode! to drop the text state when
working with templates.
Merge pull request #538 from kuroneko/master

Fix github #537 : force text rendering mode for templates
Merge pull request #535 from hbrandl/issue_502_fixed

fixes issue #502
Merge branch 'master' of https://github.com/prawnpdf/prawn
Add test case for long strings