[WebProfilerBundle] fixed output of bag values

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7540
| License       | MIT
| Doc PR        | -

Call json_encode with flags: JSON_UNESCAPED_SLASHES and
JSON_UNESCAPED_UNICODE