Fix Vulnerabilities::Finding#remediations method

What does this MR do?

This MR fixes a small issue with the Vulnerabilities::Finding#remediations method.

The problem was, Rails has a specific kwarg to use methods with as_json and if the client gives the method name within the only kwarg, it silently ignores the method name and does not add the key into the hash. We couldn't catch this problem in specs because the specs are also using the as_json method as the same way we use it in the actual implementation. This is why we shouldn't leak the implementation into our specs :/

This does not need a changelog entry since the original MR was also merged after the release cut.

Related to !50201 (merged)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team