当 response 的 Content-Encoding 是 gzip，同时 body 为空时程序崩溃

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#### Which platform are you running AnyProxy

Mac
#### The version of the AnyProxy

4.0.2

#### Your expected behavior of AnyProxy

无论什么请求，程序都正常运行
#### The actual behavior of AnyProxy

程序崩溃。
#### The log of the error

## test request：
`
curl -x 127.0.0.1:8001 'https://beacon.tingyun.com/xhr1?pvid=e90422ff-80d8-470e-8de0-407d3f6bdbec&ref=https%3A%2F%2Ftest.example.com%2F&referrer=&key=vK4oZhra2XA&v=1.7.3&av=1.7.3&did=b51f4119-bd75-4fac-8050-e512ac7eddab&sid=f095450d-a0a9-4075-9256-55120e8be309&__r=1514212434717' -H 'Pragma: no-cache' -H 'Origin: https://test.example.com' -H 'Accept-Encoding: gzip, deflate, br' -H 'Accept-Language: zh-CN,zh;q=0.9,en-US;q=0.8,en;q=0.7' -H 'User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.84 Safari/537.36' -H 'Content-Type: text/plain;charset=UTF-8' -H 'Accept: */*' -H 'Cache-Control: max-age=0' -H 'Referer: https://www.growingio.com/' -H 'Cookie: qtrans_cookie_test=qTranslate+Cookie+Test; Hm_lvt_93daecc2db29efa5d83566cf580e84af=1514212350; NTKF_T2D_CLIENTID=guest5DACA43B-1338-4E4B-FC1C-8E16DC2EE088; nTalk_CACHE_DATA={uid:kf_9212_ISME9754_guest5DACA43B-1338-4E,tid:1514212351021605}; Hm_lpvt_93daecc2db29efa5d83566cf580e84af=1514212352' -H 'Connection: keep-alive' --data-binary '{"xhr":[{"id":0,"req":"POST https://test.example.com","start":1514212433017,"du":32,"cb":0,"status":200,"err":0,"rec":0,"send":386},{"id":1,"req":"POST https://test.example.com/","start":1514212434573,"du":33,"cb":0,"status":200,"err":0,"rec":0,"send":12044}]}' --compressed
`
## Log：

[AnyProxy Log][2017-12-25 22:43:59]: received https CONNECT request beacon.tingyun.com
[AnyProxy Log][2017-12-25 22:43:59]: will forward to local https server
[AnyProxy Log][2017-12-25 22:43:59]: [internal https]proxy server for beacon.tingyun.com established
[AnyProxy Log][2017-12-25 22:43:59]: received request to: POST beacon.tingyun.com/xhr1?pvid=e90422ff-80d8-470e-8de0-407d3f6bdbec&ref=https%3A%2F%2Ftest.example.com%2F&referrer=&key=vK4oZhra2XA&v=1.7.3&av=1.7.3&did=b51f4119-bd75-4fac-8050-e512ac7eddab&sid=f095450d-a0a9-4075-9256-55120e8be309&__r=1514212434717
[AnyProxy ERROR][2017-12-25 22:43:59]: got an uncaught exception, is there anything goes wrong in your rule file ?
TypeError: Cannot read property 'length' of null
`at Gunzip.zlibBufferOnEnd (zlib.js:100:17)`
at Gunzip.emit (events.js:164:20)
at endReadableNT (_stream_readable.js:1054:12)
at _combinedTickCallback (internal/process/next_tick.js:138:11)
at process._tickCallback (internal/process/next_tick.js:180:9)
[AnyProxy Log][2017-12-25 22:43:59]: destorying https connection : 127.0.0.1:63791
[AnyProxy Log][2017-12-25 22:43:59]: endding https cltSocket : 127.0.0.1:63791
[AnyProxy Log][2017-12-25 22:43:59]: clearing cache file...
[AnyProxy Log][2017-12-25 22:43:59]: closing webserver...