`Env.__contains__` ignores configured prefix

[paduszyk]

When a prefix is set, Env.get_value uses the prefixed name, but Env.__contains__ still checks only the raw key.

Current behavior

def __contains__(self, var: _str):
    return var in self.ENVIRON

Repro

import environ

env = environ.Env()
env.prefix = "DJANGO_"
env.ENVIRON["DJANGO_DEBUG"] = "1"

env.get_value("DEBUG")  # returns "1"
"DEBUG" in env          # returns False

Expected

"DEBUG" in env should return True in this case, consistent with get_value (or simple call).

Suggested fix

def __contains__(self, var: _str):
    return f"{self.prefix}{var}" in self.ENVIRON

Happy to open a PR with this change and a test covering prefixed membership.