#15 pxssh: set_unique_promp make prompt a string

[Matthew Stoltenberg]

When setting a custom unique prompt, self.prompt is overwritten with what I was expecting to be the new prompt.

Line 126:
self.prompt = optional_prompt
Needs to be something like:
self.PROMPT = "\[%s\][\$\#] " % (optional_prompt, )
self.PROMPT_SET_SH = "PS1='[%s]\$ '" % (optional_prompt, )
self.PROMPT_SET_CSH = "set prompt='[%s]\$ '" % (optional_prompt, )