HttpHeader: add 'AddParam' function
CurlFile: use HttpHeader:AddParam
CurlFile::CReadState::HeaderCallback: internally use std::string instead of char*
Move all header parsing logic from CurlFile::CReadState::HeaderCallback to CHttpHeader::Parse
HttpHeader::GetValue: refactoring
HttpHeader::GetMimeType: fix: exclude "charset" and other secondary parameters
HttpHeader: add 'GetCharset' function
CurlFile: add "GetServerReportedCharset" function
HttpHeader: make 'GetProtoLine' inline const