Fix clippy warning

warning: this implementation is unsound, as some fields in `Connection` are `!Send`
   --> src/lib.rs:339:1
339 | unsafe impl Send for Connection {}

src/inner_connection.rs

@@ -38,6 +38,8 @@ pub struct InnerConnection {
     owned: bool,
 }
 
+unsafe impl Send for InnerConnection {}
+
 impl InnerConnection {
     #[allow(clippy::mutex_atomic)]
     #[inline]