[YAML] Parser doesn't understand properly lines with trailing spaces 

| Q                | A
| ---------------- | -----
| Bug report?      | yes
| Feature request? | no
| BC Break report? | yes
| RFC?             | no
| Symfony version  | 3.3-dev

As mentioned on https://github.com/symfony/symfony/pull/22067#issuecomment-289603202 the last commit on the yaml component breaks the parser for that scenario. 