Use insert_before for "new" entries in insert_sorted

The only difference compared to using `shift_insert` is when the
binary-searched key isn't *actually* new to the map, just not found in
properly sorted order. In this case we can't guarantee a sorted result
either, but it will at least behave better about the new position,
especially if that's the end.

src/map.rs

@@ -441,7 +441,7 @@ where
     {
         match self.binary_search_keys(&key) {
             Ok(i) => (i, Some(mem::replace(&mut self[i], value))),
-            Err(i) => (i, self.shift_insert(i, key, value)),
+            Err(i) => self.insert_before(i, key, value),
         }
     }
 

src/map/tests.rs

@@ -135,6 +135,34 @@ fn shift_insert() {
     }
 }
 
+#[test]
+fn insert_sorted_bad() {
+    let mut map = IndexMap::new();
+    map.insert(10, ());
+    for i in 0..10 {
+        map.insert(i, ());
+    }
+
+    // The binary search will want to insert this at the end (index == len()),
+    // but that's only possible for *new* inserts. It should still be handled
+    // without panicking though, and in this case it's simple enough that we
+    // know the exact result. (But don't read this as an API guarantee!)
+    assert_eq!(map.first(), Some((&10, &())));
+    map.insert_sorted(10, ());
+    assert_eq!(map.last(), Some((&10, &())));
+    assert!(map.keys().copied().eq(0..=10));
+
+    // Other out-of-order entries can also "insert" to a binary-searched
+    // position, moving in either direction.
+    map.move_index(5, 0);
+    map.move_index(6, 10);
+    assert_eq!(map.first(), Some((&5, &())));
+    assert_eq!(map.last(), Some((&6, &())));
+    map.insert_sorted(5, ()); // moves back up
+    map.insert_sorted(6, ()); // moves back down
+    assert!(map.keys().copied().eq(0..=10));
+}
+
 #[test]
 fn grow() {
     let insert = [0, 4, 2, 12, 8, 7, 11];