Fix IO.console_size to work when IO.console is not available

[PChambino]

At the moment, this works correctly when console method exists, but when it doesn't exist it actually fails with NameError instead of NoMethodError.

For example, when using JRuby on Linux aarch64 (related #40): (trimmed IRB output)

io/console on JRuby shells out to stty for most operations
irb(main):001:0> IO.console
(irb):1:in `evaluate': undefined method `console' for IO:Class (NoMethodError)
irb(main):003:0> require 'io/console/size'
irb(main):004:0> IO.console_size
uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/io/console/size.rb:19:in `console_size': undefined local variable or method `console' for IO:Class (NameError)

With the change in this PR it works as expected.

[nobu]

The receiver in IO.console_size should be IO (or a subclass of IO), and the explicit receiver IO should make no sense.
Probably sounds like an issue of JRuby implementation.