improve the phpcr-odm guesser

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | see travis
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

fixing a few situations where the guesser currently produced warnings and invalid results.