Update src/Symfony/Component/HttpFoundation/Request.php

This makes `getContentType()` work when a regular form is submitted.
feature #7142 [2.3] Update src/Symfony/Component/HttpFoundation/Request.php (datibbaw)

This PR was merged into the 2.6-dev branch.

Discussion
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[2.3] Update src/Symfony/Component/HttpFoundation/Request.php

This makes `getContentType()` work when a regular form is submitted. It would return `"form"`

Commits
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c81ec4d Update src/Symfony/Component/HttpFoundation/Request.php