Interact() does not detect EOF

Improve EOF detection from test_interact.py through echo_w_prompt.py. However this test case does not correctly test EOF condition. However, one can match this with the same routine in read_nonblocking, which is well tested. Closes issue #49 
I prefer my method of making a modified copy of `os.environ` and using it for subprocesses, rather than modifying this process's environment directly and having to change it back afterwards.