fixes #2934

Parsing dos line ending delimited line protocol is still illegal in most
cases.

This was broken by changes in go 1.8 to url.Parse.  This change allows
the string but prompts the user to move to the correct url string.

Metrics needing one extra byte to fit the output buffer would not be split, so we would emit lines without a line ending. Metrics which overflowed by exactly one field length would be split one field too late, causing truncated fields.