Fix WebHeaderCollection.GetEnumerator().

Change WebHeaderCollection.GetEnumerator() from an explicit implementation
to a public override to avoid delegating to the base collection. This change
also adds a test for this behavior.

Author: pgavlin

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

@@ -593,7 +593,7 @@ public override string[] AllKeys
             }
         }
 
-        IEnumerator IEnumerable.GetEnumerator()
+        public override IEnumerator GetEnumerator()
         {
             return InnerCollection.Keys.GetEnumerator();
         }

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

@@ -250,5 +250,21 @@ public void ToString_Success()
                 "Accept: text/plain\r\nContent-Length: 123\r\n\r\n",
                 w.ToString());
         }
+
+        [Fact]
+        public void IterateCollection_Success()
+        {
+            WebHeaderCollection w = new WebHeaderCollection();
+            w["Accept"] = "text/plain";
+            w["Content-Length"] = "123";
+
+            string result = "";
+            foreach (var item in w)
+            {
+                result += item;
+            }
+
+            Assert.Equal("AcceptContent-Length", result);
+        }
     }
 }