fix(openapi-fetch): omit serializing for already serialized data

[justAnArthur]

Changes

New changes makes omitting serializing for already serialized data, such as string, primary fixing issue with sending string as body

How to Review

Take a look at the issue: https://github.com/openapi-ts/openapi-typescript/issues/2555 and on updated code and test.

Checklist

• [x] Unit tests updated
• ~~docs/ updated (if necessary)~~
• ~~pnpm run update:examples run (only applicable for openapi-typescript)~~