HttpHeader::Parse: accept "\n" line ending as well
HttpHeader::Parse: fix parsing multiline parameter (used only in tests)
HttpHeader::GetHeader: (used only in tests) return empty string if got nothing and use standard "\r\n" line ending
HttpHeader::AddParam: fix overwrite for non-lowercase names
HttpHeader::AddParam: trim name and value like "Parse()"
HttpHeader::AddParam: fix trying to add empty value or name
TestHttpHeader: rewrite HttpHeader tests
Merge pull request #4002 from Karlson2k/httpheader_add_01

Httpheader fixes and Httpheader tests update