Don't force encoding if string is frozen

Currently HighLine forces encoding in `#say`, but that would change string in-place and incase it's frozen it would fail.

This following code doesn't work without this patch
```ruby
HighLine.new.say('Oh no'.freeze)
```
results in
```
RuntimeError: can't modify frozen String
        from /var/lib/ruby/gems/2.0.0/gems/highline-1.6.19/lib/HighLine.rb:621:in `force_encoding'
        from /var/lib/ruby/gems/2.0.0/gems/highline-1.6.19/lib/HighLine.rb:621:in `say'
```


PS. actually I think Ruby does wrong in this case it should be `force_encoding!` because it does change original string and there should be another method without `!` which would return new string.