HttpHeader: fix: parse header properly according to RFC 2616
* use 'tab' as whitespace as well as 'space'
* process header lines divided to several lines with whitespace
HttpHeader::GetMimeType: fix: trim whitespace chars according to RFC2616
HttpHeader::GetCharset: fix: properly find charset according to RFC 2616
* allow more than one whitespace
* use 'tab' for whitespace too
* allow charset to be quoted