Use copied fields to avoid byte slice was reused.

[tigerlee]

When I use the latest version(v2.2.0), I got an issue in heavy network I/O.

I'm using sse as client, when I send an event stream request to server with header Last-Event-ID: 0, server sends lots of events to client, then I found some message's fields, such as ID & Event, was not the original value.

This might be caused by a byte slice was a reference, not a value.

So I use a copied value to these fields (ID, Event, Retry) to avoid this situation.

[purehyperbole]

Hi there,

Thanks for the contribution. I'll merge this now and release!