get_mime_extensions disregards the suffix

[alecmocatta]

get_mime_extensions behaves differently to get_mime_extensions_str, in that it seems to disregard the suffix, breaking certain common mime types:

// works:
mime_guess::get_extensions("image", "svg+xml")

// fails:
mime_guess::get_mime_extensions("image/svg+xml".parse().unwrap())