Don't use natural option in tail anymore

It's not supported anymore in MongoDB 3.4.

src/collection.rs

@@ -706,14 +706,9 @@ impl<'a> Collection<'a> {
         find_options: Option<CommandAndFindOptions>,
         tail_options: Option<TailOptions>
     ) -> TailingCursor<'a> {
-        let query_with_options = doc! {
-            "$query" => query,
-            "$natural" => 1
-        };
-
         TailingCursor::new(
             self,
-            query_with_options,
+            query,
             find_options.unwrap_or(CommandAndFindOptions::default()),
             tail_options.unwrap_or(TailOptions::default())
         )

tests/cursor.rs

@@ -50,7 +50,7 @@ fn test_tailing_cursor() {
 
     // Try to tail on a normal collection
     let failing_cursor = normal_collection.tail(doc!{}, None, None);
-    let failing_result = failing_cursor.into_iter().next().unwrap();
+    let failing_result = failing_cursor.into_iter().next().expect("Nothing in iterator");
     assert!(failing_result.is_err());
 
     let document = doc! { "key_1" => "Value 1" };
@@ -75,7 +75,7 @@ fn test_tailing_cursor() {
     });
 
     // Wait for the thread to boot up
-    thread::sleep(Duration::from_millis(250));
+    thread::sleep(Duration::from_secs(1));
 
     // Insert some more documents into the collection
     for _ in 0..25 {
@@ -85,5 +85,5 @@ fn test_tailing_cursor() {
     // See if they appeared while iterating the cursor
     // The for loop returns whenever we get more than
     // 15 results.
-    assert_eq!(25, guard.join().unwrap());
+    assert_eq!(25, guard.join().expect("Thread failed"));
 }