Revert "Use round-trip format specifier for DateTime"

src/Flurl/Util/CommonExtensions.cs

@@ -37,10 +37,6 @@ public static IEnumerable<KeyValuePair<string, object>> ToKeyValuePairs(this obj
 		/// </summary>
 		public static string ToInvariantString(this object obj) {
 			// inspired by: http://stackoverflow.com/a/19570016/62600
-
-			var d = obj as DateTime?;
-			if (d != null)
-				return d.Value.ToString("O", CultureInfo.InvariantCulture);
 
 #if !NETSTANDARD1_0
 			var c = obj as IConvertible;
@@ -128,4 +124,4 @@ public static void Merge<TKey, TValue>(this IDictionary<TKey, TValue> d1, IDicti
 				d1.Add(kv);
 		}
 	}
-}
+}