- lexer.rl: accept tabs before closing heredoc delimiter

[iliabylich]

Closes #989.

The bug is not related to blocks specifically but example from the report can also be parsed now:

$ /bin/cat test.rb
foo do
	<<~DESC
		bar
	DESC
end

$ bin/ruby-parse --33 test.rb
(block
  (send nil :foo)
  (args)
  (str "bar\n"))

[iliabylich]

Turns out the latest minitest also defines BASE_DIR that conflicts with our global variable 😞