merged branch cloppy/Dom-Crawler-fix (PR #7127)

This PR was merged into the 2.0 branch.

Commits
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cb03074 [DomCrawler] lowered parsed protocol string (fixes #6986)

Discussion
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[DomCrawler] lowered parsed protocol string (fixes #6986)

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6986
| License       | MIT
| Doc PR        | symfony/symfony-docs#123

Are UTs needed for this?

src/Symfony/Component/DomCrawler/Link.php

@@ -46,7 +46,7 @@ class Link
      */
     public function __construct(\DOMNode $node, $currentUri, $method = 'GET')
     {
-        if (!in_array(substr($currentUri, 0, 4), array('http', 'file'))) {
+        if (!in_array(strtolower(substr($currentUri, 0, 4)), array('http', 'file'))) {
             throw new \InvalidArgumentException(sprintf('Current URI must be an absolute URL ("%s").', $currentUri));
         }