Add falsely passing double array roundtrip test

Sources/XMLCoder/Auxiliaries/Box/SingleElementBox.swift

@@ -14,7 +14,6 @@ struct SingleElementBox: SimpleBox {
 }
 
 extension SingleElementBox: Box {
-
     var isNull: Bool {
         return false
     }

Tests/XMLCoderTests/Minimal/BoxTreeTests.swift

@@ -28,12 +28,13 @@ class BoxTreeTests: XCTestCase {
             elements: [e1, e2],
             attributes: []
         )
-        // FIXME: Use `XCTUnwrap` when commonly available
+
         guard let boxTree = root.transformToBoxTree() as? KeyedBox else {
             XCTFail("boxtTree is not a KeyedBox");
             return
         }
         let foo = boxTree.elements["foo"]
         XCTAssertEqual(foo.count, 2)
+
     }
 }

Tests/XMLCoderTests/SimpleChoiceTests.swift

@@ -94,4 +94,18 @@ class SimpleChoiceTests: XCTestCase {
         let decoded = try XMLDecoder().decode([IntOrString].self, from: encoded)
         XCTAssertEqual(original, decoded)
     }
+
+    func testIntOrStringDoubleArrayRoundTrip() throws {
+        let original: [[IntOrString]] = [[
+            .int(1),
+            .string("two"),
+            .string("three"),
+            .int(4),
+            .int(5),
+        ]]
+        let encoded = try XMLEncoder().encode(original, withRootKey: "container")
+        print(String(data: encoded, encoding: .utf8))
+        let decoded = try XMLDecoder().decode([[IntOrString]].self, from: encoded)
+        XCTAssertEqual(original, decoded)
+    }
 }