Fix s(:arglist) creation on 1.8.7

Author: adambeynon

lib/opal/parser.rb

@@ -369,7 +369,8 @@ def new_block_args(norm, opt, rest, block)
       end
     end
 
-    def new_call(recv, meth, args = [])
+    def new_call(recv, meth, args = nil)
+      args ||= []
       sexp = s(:call, recv, value(meth).to_sym, s(:arglist, *args))
       sexp.loc = source(meth)
       sexp