AVRO-4071: put the recorde schema to the context after adding the fields

[andreabettich]

What is the purpose of the change

(For example: This pull request improves file read performance by buffering data, fixing AVRO-XXXX.) This pull request fixes the behavior when adding two records with the same name and namespace. As described in the Jira-Issue, the existing schema and the current one weren't compared correctly so it was never possible to add two identical records.

Verifying this change

Haven't added tests yet but can provide some to verify the change.

Documentation

No documentation needed