Make charset parameter name comparison case-insensitive

HTTP header parameter names are case-insensitive per RFC 7230, so a Content-Type of “text/plain; Charset=UTF-8” should be recognized the same as “charset=utf-8”. Previously only the exact lowercase form was matched, causing the charset to be ignored and the response to be decoded with the default ISO-8859-1 encoding (or the passed-in default), garbling non-ASCII text in StringRequest responses.