Ignore return_self_not_must_use clippy lint

    error: missing `#[must_use]` attribute on a method returning `Self`
       --> src/map.rs:595:5
        |
    595 | /     pub fn and_modify<F>(self, f: F) -> Self
    596 | |     where
    597 | |         F: FnOnce(&mut Value),
    598 | |     {
    ...   |
    605 | |         }
    606 | |     }
        | |_____^
        |
        = note: `-D clippy::return-self-not-must-use` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#return_self_not_must_use

    error: missing `#[must_use]` attribute on a method returning `Self`
       --> src/value/mod.rs:836:5
        |
    836 | /     pub fn take(&mut self) -> Value {
    837 | |         mem::replace(self, Value::Null)
    838 | |     }
        | |_____^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#return_self_not_must_use

src/lib.rs

@@ -313,6 +313,7 @@
     clippy::match_single_binding,
     clippy::needless_doctest_main,
     clippy::needless_late_init,
+    clippy::return_self_not_must_use,
     clippy::transmute_ptr_to_ptr,
     clippy::unnecessary_wraps,
     // clippy bug: https://github.com/rust-lang/rust-clippy/issues/5704