Fix test that failed when run with a seed of 29940

Tested-by: Mark Somerville <mark@scottishclimbs.com> [Arch, MRI 2.0.0]
Signed-off-by: Mark Somerville <mark@scottishclimbs.com>
Bring Readline.readline default args in line with C

As of Ruby 1.9, prompt, the first argument to Readline.readline,
defaults to an empty string.

Fixes #94. This was causing problems for Pry:

  https://github.com/pry/pry/issues/1048

Tested-by: Mark Somerville <mark@scottishclimbs.com> [Arch, MRI 2.0.0]
Signed-off-by: Mark Somerville <mark@scottishclimbs.com>