bugfix: Use merged request params to get request content-type

[am-burban]

Hello 👋

I just noticed that to determine the content-type of a request, only the type parameter from the request method itself is used. A type parameter provided in the baseApiParams when instantiating the HttpClient is ignored. Imho it's a good idea to pull the type from the merged request params, which also contain the global defaults.

Best, Ben