Only accept a retry value made up of ASCII digits

[adamw]

Two things were wrong with how ServerSentEvent.parse reads a retry: line. The spec says the value must be ASCII digits only, and that the field is ignored otherwise.

An unreadable value used to reset retry to None, dropping a value from an earlier line of the same event:

ServerSentEvent.parse(List("retry: 5", "retry: x")).retry  // was None, now Some(5)

A signed value used to parse, so a consumer could get a negative reconnect delay:

ServerSentEvent.parse(List("retry: -1")).retry  // was Some(-1), now None

Both now ignore the line and keep whatever was read before.

A note on the digit check: Integer.parseInt accepts non-ASCII digits, and so does Character.isDigit, so retry: ٥ parsed to Some(5). The check is an explicit '0' to '9' range for that reason. toIntOption is still needed after it, to reject a value too long to fit an Int.

Found while reviewing #478, but unrelated to it.