Change non-standard header name 'ContentMD5' to 'Content-MD5'

Fixes aspnet#833

Author: khellang

src/Microsoft.Net.Http.Headers/HeaderNames.cs

@@ -20,7 +20,7 @@ public static class HeaderNames
         public const string ContentLanguage = "Content-Language";
         public const string ContentLength = "Content-Length";
         public const string ContentLocation = "Content-Location";
-        public const string ContentMD5 = "ContentMD5";
+        public const string ContentMD5 = "Content-MD5";
         public const string ContentRange = "Content-Range";
         public const string ContentType = "Content-Type";
         public const string Cookie = "Cookie";
@@ -57,4 +57,4 @@ public static class HeaderNames
         public const string WebSocketSubProtocols = "Sec-WebSocket-Protocol";
         public const string WWWAuthenticate = "WWW-Authenticate";
     }
-}
+}