* Add init to FlaskClient
This addresses #1467. The init in the subclass
can now take in `environ_base`, which will then get
passed to `make_test_environ_builder` and to
`EnvironBuilder` via keyword args.
This should provide the default environment capability
on `app.test_client()` init.
* Add kwarg `environ_base` to `make_test_environ_builder` call
This change now passes `environ_base` from either
`kwargs` in `FlaskClient.open` or `FlaskClient.environ_base`
if passed into the init.
* Fix assignment reference typo
* Add default `environ_base` to `FlaskClient.__init__`
* Set default kwargs for `environ_base` in `FlaskClient.open`
* Remove specific environ_base kwarg since its in kwargs
* Add docstring to FlaskClient detailing environ_base
* Document app.test_client default environ in CHANGES
* Re-word environ_base changes in FlaskClient docstring
* Add client.environ_base tests
* Mention preset default environ in `app.test_client`
* Add versionchanged directive to docstring in FlaskClient
bd5e297aa9