fix default content-type

[yougyung]

Environment

fetch-ax 2.0.3 version

Reproduction

// api
await instance.post(`${API_PATH.user}/parsing-text`, body);

// error
415 Unsupported Media Type

Describe the bug

When fetch sets formdata to body without specifying the content-type, the browser automatically sets the content-type.

However, in the case of fetch-ax, an error occurs because the default value of content-type is set to application/json. I think we should remove the default value of content-type.

Additional context

Previously, the reason fetch-ax set the default value of content-type to application/json was because of the expectation that json format would be sent as the body most often.

To satisfy this intention, logic will be added to check the body when requesting and, if it is a json type, automatically set application/json to the content-type.

Logs

No response