add collection.label to Document#to_liquid
add label to document section of collections page
add collection.label to Document#to_liquid
Merge branch 'documenttoliquid' of https://github.com/budparr/jekyll into documenttoliquid

Conflicts:
lib/jekyll/document.rb
Merge branch 'documenttoliquid' of https://github.com/budparr/jekyll into documenttoliquid
change label to collection

To make it clear the label is of the document's collection
remove extra space
add test for collection name
Merge pull request #2436 from budparr/documenttoliquid