fix: add body stringification for application/json content-type

[ryota-iso]

Overview

When Content-Type: application/json is explicitly specified, automatically apply JSON.stringify to the object body to resolve the issue where [object Object] is sent.
For non-JSON Content-Types, existing behavior is preserved.

Background / Issue

issue: #76
In the current implementation, even when application/json is specified, an object body is not serialized and is instead sent as [object Object].

Key Changes

When the header includes "content-type": "application/json", JSON.stringify is applied.

Expected Behavior

For requests that specify application/json, payloads such as { "foo": "bar" } are correctly sent as a JSON string.

[bujxke]

lgtm, I've been experiencing the same issue while explicitly trying to set content-type to application/json