Adding exception message for paths not starting with / #251

Author: JunTaoLuo

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

@@ -3,6 +3,7 @@
 
 using System;
 using System.Text.Encodings.Web;
+using Microsoft.AspNet.Http.Abstractions;
 
 namespace Microsoft.AspNet.Http
 {
@@ -29,7 +30,7 @@ public PathString(string value)
         {
             if (!string.IsNullOrEmpty(value) && value[0] != '/')
             {
-                throw new ArgumentException(""/*Resources.Exception_PathMustStartWithSlash*/, nameof(value));
+                throw new ArgumentException(Resources.FormatException_PathMustStartWithSlash(nameof(value)), nameof(value));
             }
             _value = value;
         }

src/Microsoft.AspNet.Http.Abstractions/Properties/Resources.Designer.cs

@@ -132,4 +132,7 @@
   <data name="Exception_UseMiddleMutlipleInvokes" xml:space="preserve">
     <value>Multiple public '{0}' methods are available.</value>
   </data>
+  <data name="Exception_PathMustStartWithSlash" xml:space="preserve">
+    <value>The path in '{0}' must start with '/'.</value>
+  </data>
 </root>

src/Microsoft.AspNet.Http.Abstractions/Resources.resx

@@ -13,7 +13,7 @@ public class PathStringTests
         public void CtorThrows_IfPathDoesNotHaveLeadingSlash()
         {
             // Act and Assert
-            ExceptionAssert.ThrowsArgument(() => new PathString("hello"), "value", "");
+            ExceptionAssert.ThrowsArgument(() => new PathString("hello"), "value", "The path in 'value' must start with '/'.");
         }
 
         [Theory]