Support squiggly heredocs in ruby-mode

* lisp/progmodes/ruby-mode.el (ruby-here-doc-beg-re):
Support squiggly heredocs added in Ruby 2.3.

* test/indent/ruby.rb: Add squiggly example.

lisp/progmodes/ruby-mode.el

@@ -102,7 +102,7 @@
 
 (eval-and-compile
   (defconst ruby-here-doc-beg-re
-  "\\(<\\)<\\(-\\)?\\(\\([a-zA-Z0-9_]+\\)\\|[\"]\\([^\"]+\\)[\"]\\|[']\\([^']+\\)[']\\)"
+  "\\(<\\)<\\([~-]\\)?\\(\\([a-zA-Z0-9_]+\\)\\|[\"]\\([^\"]+\\)[\"]\\|[']\\([^']+\\)[']\\)"
   "Regexp to match the beginning of a heredoc.")
 
   (defconst ruby-expression-expansion-re

test/indent/ruby.rb

@@ -4,6 +4,10 @@ if something_wrong?             # ruby-move-to-block-skips-heredoc
   end
   eowarn
   foo
+
+  foo(<<~squiggly)
+  end
+  squiggly
 end
 
 def foo