Fixes HashCode issue

Author: Justin Kotalik

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

@@ -224,7 +224,7 @@ public bool Equals(PathString other)
         /// <returns>True if both PathString values are equal</returns>
         public bool Equals(PathString other, StringComparison comparisonType)
         {
-            if (!this.HasValue && !other.HasValue)
+            if (!HasValue && !other.HasValue)
             {
                 return true;
             }
@@ -251,7 +251,7 @@ public override bool Equals(object obj)
         /// <returns>The hash code</returns>
         public override int GetHashCode()
         {
-            return (_value != null ? StringComparer.OrdinalIgnoreCase.GetHashCode(_value) : 0);
+            return (HasValue ? StringComparer.OrdinalIgnoreCase.GetHashCode(_value) : 0);
         }
 
         /// <summary>

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

@@ -48,6 +48,12 @@ public void NotEquals_EmptyPathStringAndNonNullPathString()
             Assert.NotEqual(pathString, PathString.Empty);
         }
 
+        [Fact]
+        public void HashCode_CheckNullAndEmptyHaveSameHashcodes()
+        {
+            Assert.Equal(PathString.Empty.GetHashCode(), default(PathString).GetHashCode());
+        }
+
         [Theory]
         [InlineData(null, null)]
         [InlineData("", null)]