Convert port to string for SCP command building

[jhollowe]

If the port is an int, scp fails due to scp_command's final b_list trying to convert the int to a string which does not work.

This mirrors what ssh_command does.

e.g. with a port number (before fix)

>>> ret = conn.scp(("test.txt",), target="/root/test.txt")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/site-packages/openssh_wrapper.py", line 219, in scp
    scp_command = self.scp_command(filenames, target)
  File "/usr/local/lib/python3.7/site-packages/openssh_wrapper.py", line 372, in scp_command
    return b_list(cmd)
  File "/usr/local/lib/python3.7/site-packages/openssh_wrapper.py", line 46, in b_list
    return [b(item) for item in items]
  File "/usr/local/lib/python3.7/site-packages/openssh_wrapper.py", line 46, in <listcomp>
    return [b(item) for item in items]
  File "/usr/local/lib/python3.7/site-packages/openssh_wrapper.py", line 30, in b
    return string.encode('utf-8')
AttributeError: 'int' object has no attribute 'encode'

[jhollowe]

PR #13 is better, I just wanted to put in a quick fix and draw attention to this issue and that its solution is easy to implement.