Rewrite spec file

I reformatted the specs to be more intent-revealing and to use
the clear expectations (rspec-expect).

By doing this, the specs, when run with the document formatter, reveal a lot more about what this gem does:

```
ContainerNumberValidator
  passes on valid owner code and category identifier
    with NYKU561651-2
    with DRYU413526-5
    with KKFU760143-9
    with MOFU587008-7
    with KKFU137865-2
    with NYKU644101-8
    with TCLU250850-8
  fails on invalid owner code and category indentifier
    with 1CLU812975-4
    with TCLU81A975-4
    with TCLU81-975-4
    with T3LU812975-4
  fails on nil and empty values
    with nil
    with empty string
  fails on invalid checksum
    with TCLU812975-1
    with MOFU587008-2
    with DRYU413526-6
    with NYKU561651-8
  passes on mixed case valid owner code and category identifier
    with TCLU812975-4
    with TCLU8129754
    with tclu8129754
    with tclu812975-4

Finished in 0.00479 seconds
21 examples, 0 failures
```