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

tests/test_testing.py

@@ -11,6 +11,7 @@
 import pytest
 
 import flask
+import werkzeug
 
 from flask._compat import text_type
 
@@ -43,6 +44,40 @@ def test_environ_defaults():
         rv = c.get('/')
         assert rv.data == b'http://localhost/'
 
+def test_environ_base_default():
+    app = flask.Flask(__name__)
+    app.testing = True
+    @app.route('/')
+    def index():
+        flask.g.user_agent = flask.request.headers["User-Agent"]
+        return flask.request.remote_addr
+
+    with app.test_client() as c:
+        rv = c.get('/')
+        assert rv.data == b'127.0.0.1'
+        assert flask.g.user_agent == 'werkzeug/' + werkzeug.__version__
+
+def test_environ_base_modified():
+    app = flask.Flask(__name__)
+    app.testing = True
+    @app.route('/')
+    def index():
+        flask.g.user_agent = flask.request.headers["User-Agent"]
+        return flask.request.remote_addr
+
+    with app.test_client() as c:
+        c.environ_base['REMOTE_ADDR'] = '0.0.0.0'
+        c.environ_base['HTTP_USER_AGENT'] = 'Foo'
+        rv = c.get('/')
+        assert rv.data == b'0.0.0.0'
+        assert flask.g.user_agent == 'Foo'
+
+        c.environ_base['REMOTE_ADDR'] = '0.0.0.1'
+        c.environ_base['HTTP_USER_AGENT'] = 'Bar'
+        rv = c.get('/')
+        assert rv.data == b'0.0.0.1'
+        assert flask.g.user_agent == 'Bar'
+
 def test_redirect_keep_session():
     app = flask.Flask(__name__)
     app.secret_key = 'testing'