Make TLVs use maps internally. (#15)

This preserves the order of keys when serializing, which is
required by certain devices.

lib/model/tlv.js

@@ -11,12 +11,12 @@ const kTLVType_Separator = 255;
  * See Chapter 12.1
  *
  * @param {Buffer} buffer - Buffer to decode
- * @returns {Object} TLV object
+ * @returns {Map} TLV object
  */
 function decodeBuffer(buffer) {
   let position = 0;
   let lastTag = -1;
-  const result = {};
+  const result = new Map();
 
   if (!Buffer.isBuffer(buffer)) {
     return result;
@@ -27,25 +27,26 @@ function decodeBuffer(buffer) {
     const length = buffer.readUInt8(position++);
     const value = buffer.slice(position, position + length);
 
-    if (result.hasOwnProperty(tag)) {
-      if (Array.isArray(result[tag]) && tag === lastTag) {
-        const idx = result[tag].length - 1;
-        const newValue = Buffer.allocUnsafe(result[tag][idx].length + length);
-        result[tag][idx].copy(newValue, 0);
-        value.copy(newValue, result[tag][idx].length);
-        result[tag][idx] = newValue;
-      } else if (Array.isArray(result[tag])) {
-        result[tag].push(value);
+    if (result.has(tag)) {
+      const existingValue = result.get(tag);
+      if (Array.isArray(existingValue) && tag === lastTag) {
+        const idx = existingValue.length - 1;
+        const newValue = Buffer.allocUnsafe(existingValue[idx].length + length);
+        existingValue[idx].copy(newValue, 0);
+        value.copy(newValue, existingValue[idx].length);
+        existingValue[idx] = newValue;
+      } else if (Array.isArray(existingValue)) {
+        existingValue.push(value);
       } else if (tag === lastTag) {
-        const newValue = Buffer.allocUnsafe(result[tag].length + length);
-        result[tag].copy(newValue, 0);
-        value.copy(newValue, result[tag].length);
-        result[tag] = newValue;
+        const newValue = Buffer.allocUnsafe(existingValue.length + length);
+        existingValue.copy(newValue, 0);
+        value.copy(newValue, existingValue.length);
+        result.set(tag, newValue);
       } else {
-        result[tag] = [result[tag], value];
+        result.set(tag, [existingValue, value]);
       }
     } else {
-      result[tag] = value;
+      result.set(tag, value);
     }
 
     position += length;
@@ -60,13 +61,14 @@ function decodeBuffer(buffer) {
  *
  * See Chapter 12.1
  *
- * @param {Object} object - TLV object to encode
+ * @param {Map} object - TLV object to encode
  * @returns {Buffer} Encoded buffer
  */
 function encodeObject(object) {
   const tlvs = [];
 
-  for (let tag in object) {
+  // eslint-disable-next-line prefer-const
+  for (let [tag, value] of object) {
     tag = parseInt(tag, 10);
 
     if (tag < 0 || tag > 255) {
@@ -78,7 +80,6 @@ function encodeObject(object) {
       continue;
     }
 
-    const value = object[tag];
     let values;
     if (Array.isArray(value)) {
       values = value;