hang on chain if pipe nothing to awk

`delegator.chain("printf '' | awk '{print $1}'")` would hang, while
`delegator.run("printf '' | awk '{print $1}'")` wouldn't.
It seems that the command would block in `pexpect`：
```
^C---------------------------------------------------------------------------
KeyboardInterrupt Traceback (most recent call last)
in
----> 1 delegator.chain("printf '' | awk '{print $1}'")

~/.pyenv/versions/3.7.0/lib/python3.7/site-packages/delegator.py in chain(command, timeout, cwd, env)
308 c.subprocess.sendeof()
309
--> 310 data = c.out
311
312 return c

~/.pyenv/versions/3.7.0/lib/python3.7/site-packages/delegator.py in out(self)
126 self.__out = self.std_out.read()
127 else:
--> 128 self.__out = self._pexpect_out
129
130 return self.__out

~/.pyenv/versions/3.7.0/lib/python3.7/site-packages/delegator.py in _pexpect_out(self)
114 result += self.subprocess.after
115
--> 116 result += self.subprocess.read()
117 return result
118

~/.pyenv/versions/3.7.0/lib/python3.7/site-packages/pexpect/spawnbase.py in read(self, size)
439 if size < 0:
440 # delimiter default is EOF
--> 441 self.expect(self.delimiter)
442 return self.before
443

~/.pyenv/versions/3.7.0/lib/python3.7/site-packages/pexpect/spawnbase.py in expect(self, pattern, timeout, searchwindowsize, async_, **kw)
339 compiled_pattern_list = self.compile_pattern_list(pattern)
340 return self.expect_list(compiled_pattern_list,
--> 341 timeout, searchwindowsize, async_)
342
343 def expect_list(self, pattern_list, timeout=-1, searchwindowsize=-1,

~/.pyenv/versions/3.7.0/lib/python3.7/site-packages/pexpect/spawnbase.py in expect_list(self, pattern_list, timeout, searchwindowsize, async_, **kw)
367 return expect_async(exp, timeout)
368 else:
--> 369 return exp.expect_loop(timeout)
370
371 def expect_exact(self, pattern_list, timeout=-1, searchwindowsize=-1,

~/.pyenv/versions/3.7.0/lib/python3.7/site-packages/pexpect/expect.py in expect_loop(self, timeout)
111 incoming = spawn.read_nonblocking(spawn.maxread, timeout)
112 if self.spawn.delayafterread is not None:
--> 113 time.sleep(self.spawn.delayafterread)
114 if timeout is not None:
115 timeout = end_time - time.time()

KeyboardInterrupt:
```

My delegator version is `0.1.1`