Issue with encode_request and encode_urlencoded_data

[hekhuisk]

Hi, I found a couple of issues while using this library.

1. AsyncClient.post() takes in data which is supposed to be RequestData or None. Looking at the RequestData type it says Mapping[str, Any]. Then inside encode_request it also takes in data which is supposed to be RequestData or None. However, the first line of that method is checking if data is not None and not isinstance(data, Mapping) which implies it can be a type besides RequestData or None. That means a typechecker does not like you passing bytes for data, even though it is a valid type to pass (yes I know it says it is deprecated, but having something deprecated should not invalidate type hinting).

2. If you do pass a Mapping, in my case a dict, to AsyncClient.post() for the data field, and that dict happens to contain a key/value pair where the value is another dict containing a value of None, then you end up with a string that says "None" in it. The reason is because of this line in encode_urlencoded_data.

   The fix that is needed is to check if isinstance(value, Mapping) and handle properly converting the values in it to a string. You also need to recursively go through all values to check for more nested Mappings. This applies to the check for list and tuple as well where it does plain_data.extend([(key, primitive_value_to_str(item)) for item in value]). If item is a Mapping you run into the same issue.

3. I found Form-encoded data with None as the value gets passed as string "None" #1500 and see that the choice was made to always convert None to "". I've had many use cases where None is supposed to represent null and not empty string. So if I make a call out to something that can be None or a str and it receives "" it will become str instead of properly becoming None. Perhaps Request should take a flag called none_as_null that if True would put null in the serialized JSON instead of "".?