Header refinements

[lovelydinosaur]

Closes #1226

Creating a Request instance directly has neater more minimal header behaviour.
Only includes mandatory headers.

request = httpx.Request("POST", "http://example.org", json={"test": 123})

assert request.headers == httpx.Headers({
    "Host": "example.org",
    "Content-Type": "application/json",
    "Content-Length": "13",
})

Instead the default headers are on the client instance...

>>> client = httpx.Client()
>>> print(client.headers)
Headers({'accept': '*/*', 'accept-encoding': 'gzip, deflate, br', 'connection': 'keep-alive', 'user-agent': 'python-httpx/0.14.2'})

Can include additional headers...

>>> client = httpx.Client(headers={'Custom-Header': 'Custom'})
>>> print(client.headers)
Headers({'accept': '*/*', 'accept-encoding': 'gzip, deflate, br', 'connection': 'keep-alive', 'user-agent': 'python-httpx/0.14.2', 'custom-header': 'Custom'})

Or override existing headers...

>>> client = httpx.Client(headers={'User-Agent': 'Custom'})
>>> print(client.headers)
Headers({'accept': '*/*', 'accept-encoding': 'gzip, deflate, br', 'connection': 'keep-alive', 'user-agent': 'Custom'})

Defaults may be removed, but must be done as an explicit del...

>>> client = httpx.Client()
>>> del client.headers['User-Agent']
>>> print(client.headers)
Headers({'accept': '*/*', 'accept-encoding': 'gzip, deflate, br', 'connection': 'keep-alive'})

[florimondmanca]

Very nice and sweet indeed!