pick function change spec  as suggested GH-179

Fix the spec test to use expect {}.to instead of lambda {}.should
as explained by Adrienthebo.

"Using expect { }.to is preferred over lambda { }.should.
 In addition it's best practice to do a string match against the
error message to ensure that we're catching the right error,
instead of any error of the right type."

Also fixed a typo on the error message, it was missing one space.