fix(event-stream): don't send a message if we're stopping immediately

Related to mtrudel/bandit#141, we get into
this situation when we close the connection before truly entering the
hibernate loop. Instead, we close/unsubscribe directly, and return the
conn immediately.

Sample error from `api-prod` (truncated):
```
Bandit.HTTP1.Handler #PID<0.5319.7> received unexpected message in
handle_info/2: {:close, <conn>}
```

apps/api_web/lib/api_web/event_stream.ex

@@ -21,11 +21,16 @@ defmodule ApiWeb.EventStream do
 
   @spec call(Plug.Conn.t(), module, map) :: Plug.Conn.t()
   def call(conn, module, _params) do
-    state = initialize(conn, module)
-    hibernate_loop(state)
+    case initialize(conn, module) do
+      %__MODULE__{} = state ->
+        hibernate_loop(state)
+
+      %Plug.Conn{} = conn ->
+        conn
+    end
   end
 
-  @spec initialize(Plug.Conn.t(), module) :: state
+  @spec initialize(Plug.Conn.t(), module) :: state | Plug.Conn.t()
   def initialize(conn, module, timeout \\ 30_000) do
     conn =
       conn
@@ -41,8 +46,8 @@ defmodule ApiWeb.EventStream do
 
       ensure_timer(%__MODULE__{conn: conn, pid: pid, timeout: timeout})
     else
-      send(self(), {:close, conn})
-      %__MODULE__{conn: conn, pid: nil, timeout: timeout}
+      state = %__MODULE__{conn: conn, pid: nil, timeout: timeout}
+      unsubscribe(state)
     end
   end
 

apps/api_web/test/api_web/event_stream_test.exs

@@ -141,8 +141,8 @@ defmodule ApiWeb.EventStreamTest do
     } do
       CheckForShutdown.shutdown()
 
-      state = initialize(conn, @module)
-      assert {:close, conn} = receive_result(state)
+      conn = call(conn, @module, %{})
+      assert_receive {:plug_conn, :sent}
 
       assert chunks(conn) == ""
     end