Underscore('_') removal too arbitrary in _safe_attr

Consider this code:

from box import Box
b = Box(_out = 'preserved')
b.update({'out': 'updated'})
# expected:
# {'_out': 'preserved', 'out': 'updated'}
# observed:
# {'_out': 'updated'}
# out lost

I thought this was designed intentionally at the beginning. However, when I digged into the code, I found the problem was due to https://github.com/cdgriffith/Box/blob/master/box.py#L151

    out = ''
    for character in attr:
        out += character if character in allowed else "_"
    out = out.strip("_")  <- L151

This arbitrarily removed all underscores in the key, even the originally assigned ones.
I won’t say this is a bug, but really confusing.

Maybe we could use some unique characters to replace the characters that are not allowed, which will be removed later.

Say:

    out = ''
    for character in attr:
        out += character if character in allowed else "@"
    out = out.strip("@")

[cdgriffith]

After thinking about it some more, this is going to have to hold off until a major release. It is a bug fix, however any exiting code that uses entities like these would break on update. I am actively working on 4.0 and will slate it for that.

[cdgriffith]

Fixed in 4.0