Revert "Merge pull request #218 from Shopify/dont_render_blank_blocks"

This reverts commit 4a103a9dde9fa16d0b1583ad377474c0dee0edf0, reversing
changes made to 5c5e7de31e6d0da5529b642669343ab4b08afa4f.

Conflicts:
	lib/liquid/block.rb
	test/liquid/blank_test.rb