SSE handler never dropped

[fasterthanlime]

First off: thanks for v0.11.0!

Here's a repro for the issue I raised on twitter:

• https://github.com/fasterthanlime/sse-never-dies

just cargo run, it gives instructions on how to run curl in a separate terminal to see the effect.

Expected behavior

When a client disconnect, the internal SSE task gets dropped, and so does the handler Future, resulting in PrintOnDrop being dropped.

Observed behavior

The internal SSE task never stops running, the handler is never dropped, it just gets stuck on send after a while - presumably after it's filled the internal buffer of sse::Encoder?