Stricter data typechecking

[tomchristie]

Closes #3127

Before...

>>> import httpx
>>> data = {"key": {"this": "ain't json, buddy"}}
>>> 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'

After...

>>> import httpx
>>> data = {"key": {"this": "ain't json, buddy"}}
>>> r = httpx.post("https://www.example.com", data=data)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tom.christie/GitHub/encode/httpx/httpx/_api.py", line 331, in post
    return request(
  File "/Users/tom.christie/GitHub/encode/httpx/httpx/_api.py", line 118, in request
    return client.request(
  File "/Users/tom.christie/GitHub/encode/httpx/httpx/_client.py", line 825, in request
    request = self.build_request(
  File "/Users/tom.christie/GitHub/encode/httpx/httpx/_client.py", line 359, in build_request
    return Request(
  File "/Users/tom.christie/GitHub/encode/httpx/httpx/_models.py", line 342, in __init__
    headers, stream = encode_request(
  File "/Users/tom.christie/GitHub/encode/httpx/httpx/_content.py", line 235, in encode_request
    return encode_urlencoded_data(data)
  File "/Users/tom.christie/GitHub/encode/httpx/httpx/_content.py", line 167, in encode_urlencoded_data
    plain_data.append(_coerce_type(key, value))
  File "/Users/tom.christie/GitHub/encode/httpx/httpx/_content.py", line 153, in _coerce_type
    raise TypeError(
TypeError: Request data values must be str, int, float, bool, or None. Got type 'dict' for key 'key'.

I've not changed any type signatures here, and aimed to keep the PR as focused as possible.

Ideally we should be following up with more comprehensive documentation on the behaviour of content=..., data=..., files=..., json=....

[tomchristie]

Oh... thanks for the fast turnaround @zanieb. 😊