Add HBMustacheCustomRenderable

Support custom rendering of swift types
Fixes tests that rely on NSNull

Sources/HummingbirdMustache/CustomRenderable.swift

@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Hummingbird server framework project
+//
+// Copyright (c) 2021-2021 the Hummingbird authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See hummingbird/CONTRIBUTORS.txt for the list of Hummingbird authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+import Foundation
+
+protocol HBMustacheCustomRenderable {
+    var renderText: String { get }
+    var isNull: Bool { get }
+}
+
+extension HBMustacheCustomRenderable {
+    var renderText: String { String(describing: self) }
+    var isNull: Bool { false }
+}
+
+extension NSNull: HBMustacheCustomRenderable {
+    public var renderText: String { "" }
+    public var isNull: Bool { true }
+}