Merge: Allow controllers to set PDFKit-save-pdf

Conflicts:
	lib/pdfkit/middleware.rb
Merge: get first PDFKit-save-pdf test passing

Conflicts:
	lib/pdfkit/middleware.rb
Merge: PDFKit-save-pdf pdf save header tests

Conflicts:
	lib/pdfkit/middleware.rb
prevent header corruption in tests
Fix up gemspec
Add .ruby-version, .ruby-gemset

Fixes #202
update README.md for PDFKit-save-pdf.