Default environ (#2047)

* 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

CHANGES

@@ -19,6 +19,8 @@ Version 0.12
   well as error handlers.
 - Disable logger propagation by default for the app logger.
 - Add support for range requests in ``send_file``.
+- ``app.test_client`` includes preset default environment, which can now be
+  directly set, instead of per ``client.get``.
 
 Version 0.11.2
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