Remove should from specs

**WIP do not merge yet**
Make more consistent the test code. This is strongly influenced by the guideline http://betterspecs.org/.
This pull request basically does these things:
- [x] Remove the word `should` at the beginning of each test name.
- [x] Use `expect` syntax instead of `should`
- [x] Use properly `.` and `#` for test class and object methods