HttpURLConnectionDownloaderService: be less strict to content type header

[php-coder]

At present, when I'm allowing text/html, the text/html;charset=UTF-8 and text/html; charset=UTF-8 are rejected. We should treat them similarly. Let's ignore everything after ;.

Required for: #660
Could be related to: #594
Follow-up to 5fbc18b (#199)