Fixed fatal error in normalize/denormalizeObject.
merged branch linclark/7971-error-denormalizeObject (PR #7971)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #7971).

Discussion
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[Serializer] Fatal error in denormalizeObject for Normalizers that do not implement DenormalizerInterface

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 7971
| License       | MIT
| Doc PR        |

If there is a list of Normalizers, and only some of them implement DenormalizerInterface, then a call to deserialize() can result in a fatal error.

This is because denormalizeObject does not check that the Normalizer implements DenormalizerInterface before calling supportsDenormalization on it.

Commits
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cacca2f [Serializer] Fixed fatal error in normalize/denormalizeObject.
[Serializer] fixed CS (refs #7971)