Restore public surface.

Author: pgavlin

src/System.Net.WebHeaderCollection/src/System/Net/WebHeaderCollection.cs

@@ -593,6 +593,11 @@ public override string[] AllKeys
             }
         }
 
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
+
         public override IEnumerator GetEnumerator()
         {
             return InnerCollection.Keys.GetEnumerator();

src/System.Net.WebHeaderCollection/tests/WebHeaderCollectionTest.cs

@@ -266,5 +266,22 @@ public void IterateCollection_Success()
 
             Assert.Equal("AcceptContent-Length", result);
         }
+
+        [Fact]
+        public void IterateIEnumerable_Success()
+        {
+            WebHeaderCollection w = new WebHeaderCollection();
+            w["Accept"] = "text/plain";
+            w["Content-Length"] = "123";
+
+            string result = "";
+            foreach (var item in (System.Collections.IEnumerable)w)
+            {
+                result += item;
+            }
+
+            Assert.Equal("AcceptContent-Length", result);
+        }
+
     }
 }