Add test

Author: HaoK

src/Microsoft.AspNetCore.Http.Abstractions/PathString.cs

@@ -450,9 +450,7 @@ public static implicit operator PathString(string s)
         /// </summary>
         /// <param name="path"></param>
         public static implicit operator string(PathString path)
-        {
-            return path.ToString();
-        }
+            => path.ToString();
 
         internal static PathString ConvertFromString(string s)
             => string.IsNullOrEmpty(s) ? new PathString(s) : FromUriComponent(s);

test/Microsoft.AspNetCore.Http.Abstractions.Tests/PathStringTests.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
@@ -221,5 +221,15 @@ public void PathStringConvertsOnlyToAndFromString()
             Assert.False(converter.CanConvertTo(typeof(int)));
             Assert.False(converter.CanConvertTo(typeof(bool)));
         }
+
+        [Fact]
+        public void PathStringStaysEqualAfterAssignments()
+        {
+            PathString p1 = "/?";
+            string s1 = p1; // Becomes "/%3F"
+            PathString p2 = s1; // Stays "/%3F"
+            Assert.Equal(p1, p2);
+        }
+
     }
 }