Add a test for rust-lang#3139

Author: topecongiro

tests/source/type.rs

@@ -121,3 +121,11 @@ fn issue3117() {
         }
     }
 }
+
+// #3139
+fn issue3139() {
+    assert_eq!(
+        to_json_value(&None ::  <i32>).unwrap(),
+        json!(  { "test": None  ::  <i32> }  )
+    );
+}

tests/target/type.rs

@@ -120,3 +120,11 @@ fn issue3117() {
         }
     }
 }
+
+// #3139
+fn issue3139() {
+    assert_eq!(
+        to_json_value(&None::<i32>).unwrap(),
+        json!({ "test": None::<i32> })
+    );
+}