When monitoring idle connections, only call eof on tcp socket (#912)

* When monitoring idle connections, only call eof on tcp socket

In a heavy cloud storage workflow, I encountered the following error:

```
AssertionError: ctx.bytesavailable > 0 || !(ctx.isreadable)
```
with traces back to calling `eof(::SSLContext)` from `monitor_idle_connection`.

As the workflow was highly concurrent, I believe the error was just a data race
between an old async `monitor_idle_connection` task calling `eof` -> `wait_for_decrypted_data`,
but when returning from `wait_for_decrypted_data` due to a new connection being reused in HTTP.jl,
the `handshake` was redone, which resulted in the `AssertionError` shown above.

The proposal here is that when monitoring idle connections, we only call `eof` on the underlying
TCPSocket, since we're really only concerned with knowing whether the core socket gets additional
data, encrypted or not, when we don't expect it.

* simplify

src/ConnectionPool.jl

@@ -261,8 +261,8 @@ TODO: or if response data arrives when no request was sent (isreadable == false)
 """
 function monitor_idle_connection(c::Connection)
     try
-        if eof(c.io)                                  ;@debugv 3 "💀  Closed:     $c"
-            close(c.io)
+        if eof(tcpsocket(c.io))                                  ;@debugv 3 "💀  Closed:     $c"
+            isopen(c.io) && close(c.io)
         end
     catch ex
         @try Base.IOError close(c.io)