[ruby/irb] Ensure stdout is a TTY before calling winsize

When outputting a (possibly truncated) value, IRB will query the
window size.  However, if IRB was piped to another process, stdout
will no longer be a TTY and will not support the `winsize` method.

This fix ensure that stdout is a TTY.

https://github.com/ruby/irb/commit/125de5eeea

lib/irb/input-method.rb

@@ -39,7 +39,7 @@ module IRB
     public :gets
 
     def winsize
-      if instance_variable_defined?(:@stdout)
+      if instance_variable_defined?(:@stdout) && @stdout.tty?
         @stdout.winsize
       else
         [24, 80]

test/irb/test_context.rb

@@ -30,10 +30,6 @@ module TestIRB
       def reset
         @line_no = 0
       end
-
-      def winsize
-        [10, 20]
-      end
     end
 
     def setup
@@ -135,6 +131,18 @@ module TestIRB
         ], out)
     end
 
+    def test_output_to_pipe
+      input = TestInputMethod.new(["n=1"])
+      input.instance_variable_set(:@stdout, StringIO.new)
+      irb = IRB::Irb.new(IRB::WorkSpace.new(Object.new), input)
+      irb.context.echo_on_assignment = :truncate
+      out, err = capture_output do
+        irb.eval_input
+      end
+      assert_empty err
+      assert_equal "=> 1\n", out
+    end
+
     def test_eval_object_without_inspect_method
       verbose, $VERBOSE = $VERBOSE, nil
       all_assertions do |all|