std: Add regression test for rust-lang#32074

Just to make sure we don't accidentally break this in the future.

Author: alexcrichton

src/libstd/ascii.rs

@@ -669,4 +669,10 @@ mod tests {
                     &from_u32(lower).unwrap().to_string()));
         }
     }
+
+    #[test]
+    fn inference_works() {
+        let x = "a".to_string();
+        x.eq_ignore_ascii_case("A");
+    }
 }