SyntaxError: JSON.parse: unexpected non-whitespace character after JSON data at line 4 column 1 of the JSON data

复制 http://anyproxy.io/cn#规则模块（rule）修改返回内容并延迟中的脚本保存到本地，按照步骤运行
配置浏览器代理后，打开http://httpbin.org/user-agent 提示SyntaxError: JSON.parse: unexpected non-whitespace character after JSON data at line 4 column 1 of the JSON data
#### Which platform are you running AnyProxy

Ubuntu 16.04
#### The version of the AnyProxy

AnyProxy 4.0.0-beta18

#### Your expected behavior of AnyProxy

和文档结果一致
#### The actual behavior of AnyProxy

提示SyntaxError: JSON.parse: unexpected non-whitespace character after JSON data at line 4 column 1 of the JSON data
#### The log of the error

Start by reproducing the documented rule-module example against http://httpbin.org/user-agent with AnyProxy 4.0.0-beta18 on Ubuntu 16.04, then inspect the returned content and the JSON.parse error. Done means the saved example runs without the unexpected non-whitespace character error and matches the documented result.

Written by the indexing model from the issue text.