Remove line terminators in eventType/id/data

[magdzikk]

Fixes #480.

toString could emit fields the caller never set: eventType and id were interpolated as-is, and data was split on \n only. Since the WhatWG specification treats CRLF, a lone LF and a lone CR as line terminators, any of these in a value ended the line on the wire and the remainder was parsed by the client as a new data, event, id or retry field.

• data may legitimately span lines, so it now splits on all three terminators, each part emitted as its own data: line.
• eventType and id cannot span lines, so terminators are removed.