_initial_invalid_state not reset to true on searchFilesInitialize

# Issue report

## Brief description of issue

Code in `searchFilesInitialize` expects `_initial_invalid_state` to be true before making an API call. This flag is set to true in the constructor. After a first call to `searchFilesInitialize` everything works correctly, you can call afterwards `getNextResponse`, `getPreviousResponse`, etc.

However if you try to start a new search with the SearchFiles object calling again `searchFilesInitialize`, you will not be able to use `getNextResponse`, `getPreviousResponse`, etc. because `searchFilesInitialize` doesn't reset `_initial_invalid_state` to true.

## Proposed fix (if applicable)

Add `_initial_invalid_state = true;` add the end of `searchFilesInitialize`.

Start at the SearchFiles object's searchFilesInitialize entry point and inspect how _initial_invalid_state is set in the constructor and consumed by getNextResponse and getPreviousResponse. Done means repeated initialization leaves the object ready for those navigation methods while preserving the existing behavior after the first initialization.

Scritto dal modello di indicizzazione a partire dal testo della issue.