[DomCrawler] Fixed the Crawler::html() method for PHP versions earlier than 5.3.6.

Node argument was added to the DOMDocument::saveHTML() in PHP 5.3.6. See http://php.net/manual/en/domdocument.savehtml.php.
merged branch jakzal/bugfix/domcrawler-savehtml (PR #8013)

This PR was merged into the master branch.

Discussion
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[DomCrawler] Fixed the Crawler::html() method for early PHP versions

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes, but not on travis (segfault)
| Fixed tickets | #7963
| License       | MIT
| Doc PR        | -

Node argument was added to the [`DOMDocument::saveHTML()`](http://php.net/manual/en/domdocument.savehtml.php) in PHP 5.3.6. See http://php.net/manual/en/domdocument.savehtml.php.

It's not a nice looking solution, but seems to be the only option. Condition should be removed once PHP dependency goes over 5.3.6.

Commits
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a4e3ebf [DomCrawler] Fixed the Crawler::html() method for PHP versions earlier than 5.3.6.