Use class methods instead of property methods with arrow functions

[equadon]

Currently, it is not possible to subclass and override methods in many of the classes in react-searchkit because some methods are property methods with arrow functions which can't be inherited.

Some examples of classes with these issues:

• lib/api/contrib/invenio/InvenioRequestSerializer.js
• lib/api/contrib/invenio/InvenioResponseSerializer.js

These methods probably should be converted to normal methods in the class and bind this in the constructor.