siteCmd command did not work

### Step 1: Please describe your environment

* ZeroNet version: 4053
* Operating system: Ubuntu 18.04
* Web browser: Vivaldi
* Tor status: not available
* Opened port: yes

### Step 2: Describe the problem:
```
bitcoren@name:~/ZeroNet-py3$ python3 zeronet.py siteCmd 1KoLHoz1nSddbuf6HbfP19F7Ht7xNBSa6m fileGet content.json
- Starting ZeroNet...
- Libsecpk256k1 loaded
- Version: 0.7.0 r4053, Python 3.6.7 (default, Oct 22 2018, 11:32:17)
[GCC 8.2.0], Gevent: 1.4.0
FileServer IPv6 not supported: [Errno 101] Network is unreachable
- Unhandled exception: Expecting value: line 1 column 1 (char 0)
Traceback (most recent call last):
File "zeronet.py", line 23, in main
main.start()
File "/home/bitcoren/ZeroNet-py3/src/main.py", line 515, in start
actions.call(config.action, action_kwargs)
File "/home/bitcoren/ZeroNet-py3/src/main.py", line 111, in call
back = func(**kwargs)
File "/home/bitcoren/ZeroNet-py3/src/main.py", line 333, in siteCmd
res = json.loads(ws.recv())
File "/usr/lib/python3.6/json/__init__.py", line 354, in loads
return _default_decoder.decode(s)
File "/usr/lib/python3.6/json/decoder.py", line 339, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib/python3.6/json/decoder.py", line 357, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```

从 src/main.py:333 的 siteCmd 开始，跟踪针对所提供命令的 ws.recv() 响应。在报告的环境中重现该命令；当 siteCmd 在不出现 JSONDecodeError 的情况下处理该响应并返回所请求文件的结果时，即表示完成。

由索引模型根据 Issue 内容生成。