Taboola Bid Adapter: fix ortb2 user override issue

[ahmadlob]

Type of change

• [x] Bugfix

• [ ] Feature

• [ ] New bidder adapter

• [ ] Updated bidder adapter

• [ ] Code style update (formatting, local variables)

• [ ] Refactoring (no functional changes, no api changes)

• [ ] Build related changes

• [ ] CI related changes

• [ ] Does this change affect user-facing APIs or examples documented on http://prebid.org?

• [ ] Other

Description of change

user object entities in first party data (ortb2) not passed within the server generated request. In this PR, fixed the override issue and passing what the ortb convertor generated "user" field as it is

Other information

[ahmadlob]

@spotxslagle added more tests, thanks!

[ahmadlob]

@ChrisHuie When approximately this PR will be merged? thanks @spotxslagle