Mutable default values for arguments

Package version (if known): 1.0.0

## Describe the bug
It looks like there's some mutable objects used as default values for some function arguments, e.g. a [dictionary literal here](https://github.com/inveniosoftware/flask-resources/blob/master/flask_resources/serializers/base.py#L48).
Chances are that this is [unintentional and basically a pitfall](https://towardsdatascience.com/python-pitfall-mutable-default-arguments-9385e8265422) that I think every Python programmer has fallen into at some point.

Unless this is intended, we should change the default values for arguments to something non-mutable.

Start with flask_resources/serializers/base.py around line 48, then inspect the package for other function arguments using mutable default values. Done means the unintended mutable defaults have been replaced with non-mutable defaults and the existing test suite passes.

Written by the indexing model from the issue text.