Add Explicite ContentType Header with JSON value to KTOR clients

[Marek00Malik]

Fix that addresses the issue discussed in PR nr #12966. Adding missing ContentType header with JSON value.

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[Marek00Malik]

@wing328 can we merge this PR? We would love to start using Ktor clients within our Ktor projects and not add additional unnecessary dependencies.

[wing328]

@Marek00Malik can you please take a look at the error in https://github.com/OpenAPITools/openapi-generator/actions/runs/3244612257/jobs/5321013628 ?

[Marek00Malik]

@Marek00Malik can you please take a look at the error in https://github.com/OpenAPITools/openapi-generator/actions/runs/3244612257/jobs/5321013628 ?

Hey @wing328. Samples fixed