fixed ticket 7210
merged branch matthijsvandenbos/matthijsvandenbos/ticket_7210 (PR #7211)

This PR was submitted for the 2.1 branch but it was merged into the master branch instead (closes #7211).

Commits
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41abfd8 [DomCrawler] fixed ticket 7210

Discussion
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[DomCrawler] fixed ticket 7210: DomCrawler is not properly extendable

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7210
| License       | MIT
| Doc PR        |

See https://github.com/symfony/symfony/issues/7210 for a description of the argument