Minor issues with field2choices

Nobody has claimed this yet.

I think that in marshmallow swagger, some pathological cases are not handled the best way.

This

```python
field = fields.Str(validate=[
validate.Equal('only choice'),
validate.Equal('only choice_2')])
```
produces

```
['enum'] = ['only choice', 'only_choice_2']
```

which is wrong. Admittedly, the field declaration does not make much sense.

Also

```python
field = fields.Str(validate=[
validate.Equal('only choice'),
validate.OneOf(['freddie', 'brian', 'john'])])
```

produces

```
['enum'] = ['only choice']
```

which is wrong too. This field declaration is a big dodgy as well.

Those are corner cases that might happen, perhaps, in some complex inheritance scenarios.

apispec should at least return the correct output. Maybe raising a warning is out of scope, as if we engage in consistency validation in apispec, this could end up being a lot of work.

No time and interest to address this right now but I thought I'd leave a note here.

Start at the field2choices logic and reproduce the two validator examples from the issue. Check how multiple Equal validators and a combination of Equal with OneOf are converted into an enum. Done means both cases produce the correct enum output; the payload does not name a test file.

Written by the indexing model from the issue text.