Allow the constructor for ProductHeaderValue to accept a framework …

…version of the object

Octokit/Http/ProductHeaderValue.cs

@@ -1,4 +1,6 @@
-namespace Octokit
+using System;
+
+namespace Octokit
 {
     /// <summary>
     /// Represents a product header value. This is used to generate the User Agent string sent with each request. The
@@ -38,9 +40,16 @@ public ProductHeaderValue(string name, string version)
         {
         }
 
-        ProductHeaderValue(System.Net.Http.Headers.ProductHeaderValue productHeader)
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ProductHeaderValue"/> class.
+        /// </summary>
+        /// <remarks>
+        /// See more information regarding User-Agent requirements here: https://developer.github.com/v3/#user-agent-required
+        /// </remarks>
+        /// <param name="name">The <see cref="System.Net.Http.Headers.ProductHeaderValue"/>.</param>
+        public ProductHeaderValue(System.Net.Http.Headers.ProductHeaderValue productHeader)
         {
-            _productHeaderValue = productHeader;
+            _productHeaderValue = productHeader ?? throw new ArgumentNullException(nameof(productHeader));
         }
 
         /// <summary>