Error getting subprocess error with non-blocking mode

Accessing `command.err` a second time causes an exception

```
In [1]: import delegator

In [2]: command = delegator.run('echo a', block=False)

In [3]: command.err
Out[3]: 'a\n'

In [4]: command.err
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
in ()
----> 1 command.err

~/.virtualenvs/service-mailer/lib/python3.6/site-packages/delegator.py in err(self)
114 return self.__err
115 else:
--> 116 return self._pexpect_out
117
118 @property

~/.virtualenvs/service-mailer/lib/python3.6/site-packages/delegator.py in _pexpect_out(self)
82 result += self.subprocess.before
83 if self.subprocess.after:
---> 84 result += self.subprocess.after
85
86 result += self.subprocess.read()

TypeError: must be str, not type
```

The cause is that `self.subprocess.after` becomes `` after the first `command.err`