Undeclared header error

This now assumes `header-present` for `text/csv` mime types, if the header is not specified in some other way. That means that the `:undeclared_header` error will not be thrown for basic CSV publishing without special headers; instead you will get an `:assumed_header` info message. `:undeclared_header` will now only be shown if the MIME type is wrong and a header declaration is not found anywhere else.