Refine headers behaviour for Request() and client.build_request()

[tomchristie]

Currently Request() auto-generates a bunch of headers on __init__. Having thought about it for a bit I can see some refinements we could make here to:

• Give users a bit more control over the default headers, such as allowing them to remove the User-Agent header.
• Make Request() more generally useful, so that it only auto sets mandatory headers.

Rather than set the following headings on Request, we should instead include them on client.headers...

• User-Agent: python-httpx/1.2.3
• Accept: */*
• Accept-Encoding: gzip, deflate
• Connection: keep-alive

When passing Client(headers={...}) or setting client.headers = {} we should give precedence to explicitly set headers, but should include these defaults if they're not otherwise set.

# Create a client, with a custom Accept header.
# Other automatic headers are still included.
client = httpx.Client(headers={'Accept': 'application/json, */*;q=0.8'}

Or...

client = httpx.Client()

# Set a custom Accept header.
# Other automatic headers are still included.
client.headers = {'Accept': 'application/json, */*;q=0.8'}

If a user wants to actually remove one of these, they can do so by explicitly deleting it, eg...

# Create a client, without a User-Agent header.
client = httpx.Client()
del client.headers['User-Agent']

The recommended way of creating a request instance will continue to be client.build_request(...), but we'll want to highlight that more clearly, as instantiating a plain Request() instance will then give you a more basic set of behaviour by only auto-setting mandatory headers, and leaving everything else up to the user...

• Set a Host header, if the request URL is an absolute URL.
• Requests that mandate a body will always include either a Content-Length or Transfer-Encoding header.

Both of those cases will defer to any explicitly included header if one exists, and only auto set if those mandatory headers are omitted.

Also .prepare_headers() should become a private method.

This all ends up being beneficial, because users still get all the regular convenience of client.build_request(), but there's also a plainer style for creating Request() instances with complete control over the headers which doesn't make any assumptions about auto-including a set of default headers.

(Somewhat refs #1091)