Omit "should" in spec descriptions

The rationale is this: the spec string describes the expected behavior
unconditionally. The code examples, on the other hand, set up an expectation
that is tested with the call to the expect method. The code examples can
violate the expectation, but the spec string does not. The value of the spec
string is as clearly as possible describing the behavior. Including “should”
in that description adds no value. (From http://rubyspec.org/style_guide/)