Fix Video.toString() output illegal json and refactor

[Xcyq]

What problem does this PR solve?

1. Fixes the issue where Video.toString() outputs an illegal JSON string. All samples should output valid JSON, but in the example below, line 3 is incorrect:

17:11:13.976 [main] INFO com.iluwatar.partialresponse.App -- Retrieving full response from server:-
17:11:13.985 [main] INFO com.iluwatar.partialresponse.App -- Get all video information:
17:11:14.014 [main] INFO com.iluwatar.partialresponse.App -- {"id": 1,"title": "Avatar","length": 178,"description": "epic science fiction film","director": "James Cameron","language": "English",}
17:11:14.015 [main] INFO com.iluwatar.partialresponse.App -- ----------------------------------------------------------
17:11:14.015 [main] INFO com.iluwatar.partialresponse.App -- Retrieving partial response from server:-
17:11:14.015 [main] INFO com.iluwatar.partialresponse.App -- Get video @id, @title, @director:
17:11:14.022 [main] INFO com.iluwatar.partialresponse.App -- {"id": 3,"title": "Interstellar","director": "Christopher Nolan"}
17:11:14.023 [main] INFO com.iluwatar.partialresponse.App -- Get video @id, @length:
17:11:14.023 [main] INFO com.iluwatar.partialresponse.App -- {"id": 3,"length": 169}

2. Refactors FieldJsonMapper.toJson for better readability (possibly).

[Ritabrata1080]

Please add an extra line at EOF to satisfy github

[Ritabrata1080]

Please squash the commits

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[Ritabrata1080]

LGTM