Add a user_agent Session parameter to be able to override the default one.

I tried `Client` constructor, update_record method or even Session but with no luck.

I was trying to override the User-Agent
```
headers={'User-Agent': 'backoffice {}'.format(version)}
```

Start by tracing the Client constructor, update_record method, and Session handling mentioned in the issue to find where the default User-Agent is set. Done means a user_agent Session parameter can override that default, with a request-level test confirming the resulting header.

Written by the indexing model from the issue text.