Use KeyedBox like we used to (#46)

Author: jsbean

Sources/XMLCoder/Auxiliaries/XMLCoderElement.swift

@@ -51,7 +51,7 @@ struct XMLCoderElement: Equatable {
         elements.append(element)
     }
 
-    func transformToBoxTree() -> Box {
+    func transformToBoxTree() -> KeyedBox {
         let attributes = KeyedStorage(self.attributes.map { attribute in
             (key: attribute.key, value: StringBox(attribute.value) as SimpleBox)
         })

Sources/XMLCoder/Auxiliaries/XMLStackParser.swift

@@ -23,7 +23,7 @@ class XMLStackParser: NSObject {
         errorContextLength length: UInt,
         shouldProcessNamespaces: Bool,
         trimValueWhitespaces: Bool
-    ) throws -> Box {
+    ) throws -> KeyedBox {
         let parser = XMLStackParser(trimValueWhitespaces: trimValueWhitespaces)
 
         let node = try parser.parse(

Tests/XMLCoderTests/Minimal/BoxTreeTests.swift

@@ -29,10 +29,7 @@ class BoxTreeTests: XCTestCase {
             attributes: []
         )
 
-        guard let boxTree = root.transformToBoxTree() as? KeyedBox else {
-            XCTFail("boxtTree is not a KeyedBox")
-            return
-        }
+        let boxTree = root.transformToBoxTree()
         let foo = boxTree.elements["foo"]
         XCTAssertEqual(foo.count, 2)
     }