Do not call stty on non-tty

When testing on a non-tty (e.g.: github actions),
either one of these lines produces an error:

```ruby
input.echo = true
ask("question", "y") { |q| q.readline = true }
```

The error is produced by ruby internals:

```bash
echo | rake | cat

......stty: stdin isn't a terminal......
```

This change skips the stty calls if the input is not on a tty, so the
errors will not be produced.

2 files changed, 5 insertions, 3 deletions

diff --git a/lib/highline/terminal.rb b/lib/highline/terminal.rb
index 550c0c7..bfa0d08 100644
--- a/lib/highline/terminal.rb
+++ b/lib/highline/terminal.rb
@@ -176,7 +176,7 @@ class HighLine
     # Saves terminal state using shell stty command.
     def save_stty
       @stty_save = begin
-                     `stty -g`.chomp
+                     `stty -g`.chomp if input.tty?
                    rescue StandardError
                      nil
                    end
diff --git a/lib/highline/terminal/unix_stty.rb b/lib/highline/terminal/unix_stty.rb
index 1dbed36..df1460b 100644
--- a/lib/highline/terminal/unix_stty.rb
+++ b/lib/highline/terminal/unix_stty.rb
@@ -20,7 +20,9 @@ class HighLine
         rescue LoadError
         end
 
-        if /solaris/ =~ RUBY_PLATFORM &&
+        if !@output.tty?
+          [80, 24]
+        elsif /solaris/ =~ RUBY_PLATFORM &&
            `stty` =~ /\brows = (\d+).*\bcolumns = (\d+)/
           [Regexp.last_match(2), Regexp.last_match(1)].map(&:to_i)
         elsif `stty size` =~ /^(\d+)\s(\d+)$/
@@ -33,7 +35,7 @@ class HighLine
       # (see Terminal#raw_no_echo_mode)
       def raw_no_echo_mode
         save_stty 
-        system "stty raw -echo -icanon isig"
+        system "stty raw -echo -icanon isig" if input.tty?
       end
 
       # (see Terminal#restore_mode)