Stricter type checking on request data.

[tomchristie]

• Initially raised as discussion Httpx behaving differently than requests? #3118

Here's a gnarly thing...

data = {"key": {"this": "ain't json, buddy"}}
httpx.post("https://www.example.com", data=data)

Current behavior is that we'll coerce the value to a string, because we're overly lenient on the types...

>>> r = httpx.post("https://www.example.com", data=data)
>>> r.request.content
b'key=%7B%27this%27%3A+%22ain%27t+json%2C+buddy%22%7D'

Ideally we ought to raise a type error instead...

TypeError: Request data values must be str, int, float, bool, or None. Got type 'dict' for key 'key'.

(For comparison requests also leniently allows this, and also ends up sending unexpected data.)

[tomchristie]

Superseeded by #3176