RequestsMock.add() and .upsert() parameter names are different

It's just a little pain point, but `.add()` uses `method` while `.upsert()` uses `method_or_response` for the HTTP method parameter name. Both can use the same actual values, but the formal names are different.

Why is this a problem (at least for me)? I will sometimes send in the inputs by applying `**` to a dict. If the formal parameter names are different, it's a PITA to change the dict keys if I have to swap `.add()` vs. `.upsert()`.

```python
arg_dict = {
"method": "GET",
"url": "https://www.cnn.com/",
# ...
}
responses.add(**arg_dict) # this works
responses.upsert(**arg_dict) # this doesn't
```

If you think it's worthwhile I can create a little PR to change it.