Update EventSource to support chunked data

[jlkalberer]

This code doesn't work when a line comes in but it hasn't been terminated with a new line (chunked data). In that case, the split('\n') call will give you some junk data which the rest of the logic can't handle.

There might be another edge case where the XHR request finishes before the full line comes in but that's outside the scope of this change

[jordanbyron]

Hey @jlkalberer! Thanks for the pull request. The code looks like it would work. I really wish we had some tests to prove that it works, but that's not your fault, it is mine. I'm going to merge this in and hope for the best.