Update MediaManager.php

[mrcmorales]

Subject

I am targeting this branch, because {reason}.

Closes #1870.

Changelog

### Fixed
- `MediaManager` implements `MediaManagerInterface`

[phansys]

Good job @mrcmorales!
Could you please add a test in order to avoid regressions?

[VincentLanglet]

Good job @mrcmorales!
Could you please add a test in order to avoid regressions?

Can we still merge this @phansys ? It would be nice to avoid blocking a bugfix like this.

[core23]

Could you please add a test in order to avoid regressions?

Do we need tests that a class contains some interface? oO

[phansys]

Can we still merge this @phansys ? It would be nice to avoid blocking a bugfix like this.

Sure, we could add some tests later.

Do we need tests that a class contains some interface? oO

If this is considered as bugfix, I think so. A call to assertInstanceOf() should be enough IMO.

[core23]

If this is considered as bugfix, I think so. A call to assertInstanceOf() should be enough IMO.

A test should cover behavior not the design of a class.