Cannot easily submit forms after filling them due to missing handle to the form

I have a simple login page and I do

```
b.fill('login', self.username)
b.fill('password', self.password)
```

In order to submit that form I need to know details I don't care about, such as the button's name or the fact which button to click in case there are a few.

I expected to get a handle to the underlying form, maybe like this:

```
b.fill_form({'login': self.username, 'password', self.password}).submit()
```