android-切换上下文后，driver.source数据结构由json-变为xml

* **Macaca Version**:2.0.9

* **Macaca Doctor Logs**:
macaca-doctor version: 2.0.2

Node.js checklist:

node env: /usr/local/bin/node
node version: v6.10.2

iOS checklist:

Xcode Command Line Tools is ready, version: 2347.
xcodebuild version: 8.3.3
iproxy[usbmuxd] is installed at: `/usr/local/bin/iproxy`
ios_webkit_debug_proxy is installed at: `/usr/local/bin/ios_webkit_debug_proxy`
carthage is installed, version: 0.23.0

Android checklist:

JAVA version is `1.8.0_121`
JAVA_HOME is set to `/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home`
ANDROID_HOME is set to `/Users/joko/Library/Android/sdk`
Platforms is set to `/Users/joko/Library/Android/sdk/platforms/android-25`
ADB tool is set to `/Users/joko/Library/Android/sdk/platform-tools/adb`
gradle is installed, version: 3.5

Installed driver list:

android: 2.0.36
electron: 1.1.6 [out-of-date]
ios: 2.0.23 [out-of-date]

* **Operate System**:
mac os 10.12.5

* **Programming Language**: python 2.7

预置环境:android 5.0 webview版本：54.0.2840.85
步骤：切换上下文到webview。然后再切回到native。此时调用driver.source。查看driver.source数据结构
问题描述：此时数据结构类型是xml。再未执行切换到webview->native步骤前，driver.source数据结构是json，请帮忙修复此问题