[DomCrawler] [HttpFoundation] Make `Content-Type` attributes identification case-insensitive

According to [section 3.7 of RFC 2616][], media-type attribute names in the `Content-Type` header are case-insensitive.
Therefore, identification of the `text` type and the `charset` parameter in the `Content-Type` header should be case-insensitive.

[section 3.7 of RFC 2616]: http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.7