Resolve type_repetition_in_bounds clippy lint

    error: this type has already been used as a bound predicate
       --> src/value/ser.rs:278:9
        |
    278 |         T: Display,
        |         ^^^^^^^^^^
        |
        = note: `-D clippy::type-repetition-in-bounds` implied by `-D clippy::pedantic`
        = help: consider combining the bounds: `T: Sized + Display`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#type_repetition_in_bounds

    error: this type has already been used as a bound predicate
       --> src/value/ser.rs:611:9
        |
    611 |         T: Display,
        |         ^^^^^^^^^^
        |
        = help: consider combining the bounds: `T: Sized + Display`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#type_repetition_in_bounds

src/value/ser.rs

@@ -273,9 +273,9 @@ impl serde::Serializer for Serializer {
         })
     }
 
-    fn collect_str<T: ?Sized>(self, value: &T) -> Result<Value>
+    fn collect_str<T>(self, value: &T) -> Result<Value>
     where
-        T: Display,
+        T: ?Sized + Display,
     {
         Ok(Value::String(value.to_string()))
     }
@@ -606,9 +606,9 @@ impl serde::Serializer for MapKeySerializer {
         Err(key_must_be_a_string())
     }
 
-    fn collect_str<T: ?Sized>(self, value: &T) -> Result<String>
+    fn collect_str<T>(self, value: &T) -> Result<String>
     where
-        T: Display,
+        T: ?Sized + Display,
     {
         Ok(value.to_string())
     }