Fix parameters forwarding to IO#gets in ARGF.gets and Kernel#gets

Author: andrykonchin

src/main/ruby/truffleruby/core/argf.rb

@@ -283,9 +283,9 @@ def getc
     # The mechanism does track the line numbers,
     # and updates $. accordingly.
     #
-    def gets(sep = $/)
+    def gets(...)
       while advance!
-        line = @stream.gets(sep)
+        line = @stream.gets(...)
 
         unless line
           return nil if @use_stdin_only

src/main/ruby/truffleruby/core/kernel.rb

@@ -337,8 +337,8 @@ def getc
   end
   module_function :getc
 
-  def gets(*args)
-    line = ARGF.gets(*args)
+  def gets(...)
+    line = ARGF.gets(...)
     Primitive.io_last_line_set(Primitive.caller_special_variables, line) if line
     line
   end