Maps request attributes to environment variables. This works not only
for the Werzeug request object, but also any other class with an
environ attribute:
>>> class Test(object):
... environ = {'key': 'value'}
... test = environ_property('key')
>>> var = Test()
>>> var.test
'value'
If you pass it a second value it's used as default if the key does not
exist, the third one can be a converter that takes a value and converts
it. If it raises :exc:`ValueError` or :exc:`TypeError` the default value
is used. If no default value is provided `None` is used.
Per default the property is read only. You have to explicitly enable it
by passing ``read_only=False`` to the constructor.