Auto headers inserted into headers._list while not into headers._dict #1203

>>> import httpx
>>> client = httpx.Client()
>>> request = client.build_request("GET", "https://google.com")
>>> requests.headers
Headers({'host': 'google.com', 'user-agent': 'python-httpx/0.14.1', 'accept': '*/*', 'accept-encoding': 'gzip, deflate, br', 'connection': 'keep-alive'})
>>> "host" in request.headers
False
>>> request.headers.pop("host")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\programs\anaconda3\lib\_collections_abc.py", line 801, in pop
    del self[key]
  File "E:\projects\pycharm\httpx\httpx\_models.py", line 536, in __delitem__
    del self._dict[del_key]
KeyError: b'host'