Allow params={'pretty':None} to send query arg withOUT an equal sign

[jamshid]

I guess this is a followup to #1261. Please reconsider this:

https://www.python-httpx.org/compatibility/

Query Parameters
requests omits params whose values are None (e.g. requests.get(..., params={"foo": None})). This is not supported by HTTPX.

IMO it would be good and fine to break requests compatibility here.
I doubt many put a no-op "foo":None entry into params when using requests and it's likely nobody has on httpx 0.x.

Query args without an equal sign are in wide usage. E.g. elasticsearch's apis with ?pretty to format the json response or AWS' apis:
https://docs.aws.amazon.com/AmazonS3/latest/userguide/RESTAuthentication.html#ConstructingTheCanonicalizedResourceElement

If the request addresses a subresource, such as ?versioning, ?location, ?acl, ?torrent, ?lifecycle, or ?versionid, append the subresource, its value if it has one, and the question mark.

It's true most apis treat ?foo like ?foo= or ?foo=true but there are apis that treat ?foo= like ?foo=false.
Yes clients probably should give query args explicit values but sometimes you want to use or test an api with a specific url, and httpx should let you construct any legal url.

This is the current requests-compatible but imo undesirable behavior:

$ python3
>>> import json
>>> import httpx
>>> payload = {'key1': 'value1', 'key2': None, 'key3':''}
>>> r = httpx.get('https://httpbin.org/get', params=payload)
>>> print(json.dumps(r.json(), indent=4))
{
    "args": {
        "key1": "value1",
        "key2": "",
        "key3": ""
    },
...
    "url": "https://httpbin.org/get?key1=value1&key2=&key3="
}

Desired behavior:

...
    "url": "https://httpbin.org/get?key1=value1&key2&key3="
}