Add type attribute to Document instances

[ashmaroli]

• This is a 🙋 feature or enhancement.
• I've added tests
• The test suite passes locally

Summary

• Maintains consistency between Jekyll::Page and Jekyll::StaticFile where instances the latter two classes respond to the :type method.
• Optimizes Front Matter Defaults for Documents by not needing to check for both @doc.collection.label and @doc.collection.label.to_sym as is done currently

[ashmaroli]

Profiler summary

--- master branch https://travis-ci.org/jekyll/jekyll/jobs/532959217
+++ PR branch     https://travis-ci.org/jekyll/jekyll/jobs/533140377

- Total allocated: 532.65 MB (5301707 objects)
- Total retained:  18.34 MB (96237 objects)
+ Total allocated: 532.4 MB (5296981 objects)
+ Total retained:  18.33 MB (95977 objects)

[mattr-]

Was collection.label already a symbol? If not, is our replacing it with a symbol here safe?

[ashmaroli]

Yes, it is safe because, it is expected (from the comments):

jekyll/lib/jekyll/frontmatter_defaults.rb

Lines 51 to 70 in b82e4ea

	# Finds a default value for a given setting, filtered by path and type
	#
	# path - the path (relative to the source) of the page,
	# post or :draft the default is used in
	# type - a symbol indicating whether a :page,
	# a :post or a :draft calls this method
	#
	# Returns the default value or nil if none was found
	def find(path, type, setting)
	value = nil
	old_scope = nil
	matching_sets(path, type).each do |set|
	if set["values"].key?(setting) && has_precedence?(old_scope, set["scope"])
	value = set["values"][setting]
	old_scope = set["scope"]
	end
	end
	value
	end

[mattr-]

@jekyllbot: merge +minor