Support `data` with list of tuples to `data` with dictionary

The goal is to support this transformation:

```python
client.post("/", data=[('ha", "1"), ("ha", "2")])
```
To:
```python
client.post("/", data={"ha": ["1", "2"]})
```

Start by locating the client.post implementation and tracing how its data argument is normalized before the request is sent. Confirm the tuple-list form and dictionary-with-list-values form both produce the intended request data, then add or update coverage for the transformation.

Written by the indexing model from the issue text.