Update Crawler.php

If HTML is not well-formed XML parsing goes wrong.
merged branch spil-giacomo/patch-1 (PR #7963)

This PR was merged into the master branch.

Discussion
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Update Crawler.php

If HTML is not well-formed XML parsing goes wrong.
Could be useful create another method returning XML and XHTML in a `well-formed` standard?

Commits
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0a26641 Update Crawler.php
[DomCrawler] fixed tests for previous merge