Don't document non-schema object examples

[spaceraccoon]

As per #884, grape-swagger adds the example value even for non-schema object parameters, which leads to invalid OpenAPI during validation. I.e., the example keyword should not appear in any parameters object, only in any definitions schema objects i.e. body type parameters.

This error appears in Swagger editor/validators when a parameter object has an example key.

This adds a check such that example is only added then and fixes a previous spec.

[olivier-thatch]

@numbata I believe this can be closed in favor of #951.

[numbata]

@olivier-thatch, thanks for the PR! I agree with the changes, but I don’t have permission to close it.
@LeFnord, could you please take a look and close when you have a moment?