Use Networks.get with 2 args when possible

lib/hdprivatekey.js

@@ -512,7 +512,7 @@ HDPrivateKey.prototype.inspect = function() {
  */
 HDPrivateKey.prototype.toObject = function toObject() {
   return {
-    network: Network.get(BufferUtil.integerFromBuffer(this._buffers.version)).name,
+    network: Network.get(BufferUtil.integerFromBuffer(this._buffers.version), 'xprivkey').name,
     depth: BufferUtil.integerFromSingleByteBuffer(this._buffers.depth),
     fingerPrint: BufferUtil.integerFromBuffer(this.fingerPrint),
     parentFingerPrint: BufferUtil.integerFromBuffer(this._buffers.parentFingerPrint),