PATCH request overrides content type causing errors

# Expected Behavior

Vault API Patch endpoint requires 'Content-Type: application/merge-patch+json' header, otherwise it fails with an error.
HashiCorp.Vault client has AsynchronousClient.PatchAsync method, which should run successfully, if HeaderParameters include Content-Type set to application/merge-patch+json.

# Current Behavior

Currently ApiClient will overwrite content-type during patch call here:
https://github.com/hashicorp/vault-client-dotnet/blob/main/src/Vault/Client/ApiClient.cs#L466

Content-Type is initially loaded from HeaderParameters here: https://github.com/hashicorp/vault-client-dotnet/blob/main/src/Vault/Client/ApiClient.cs#L440 however its not really used in the case where PatchAsync method is called.

# Failure Information

HashiCorp.Vault 0.3.0
net8.0

## Steps to Reproduce

Following code will fail with the incorrect content type error, because the content type added through HeaderParameters is not used:

```
var data = "{ \"data\": { \"test\": \asdasdasd\" } }";
var requestOptions = new RequestOptions { Data = data };
requestOptions.HeaderParameters.Add("Content-Type", "application/merge-patch+json");
await vaultClient.Secrets.AsynchronousClient.PatchAsync(path, options);
```

# Additional Information

It's quite visible from the NewRequest method where things go wrong. Its also visible in the documentation https://developer.hashicorp.com/vault/docs/commands/patch that specific content type is required for the method to succeed.