Spec for String#start_with? for empty strings.

When using String#start_with?() with an empty search string (e.g.
"foo".start_with?('')) the method should return true. This is already the case
on MRI 2.1.2 and Rubinius 2.2.10 but there was no explicit spec for it.
Merge pull request #285 from YorickPeterse/master

Spec for String#start_with? for empty strings.