fix(api): 修复 CreateAssetApi 使用的错误请求头

simbot-component-kook-api/src/commonMain/kotlin/love/forte/simbot/kook/api/asset/CreateAssetApi.kt

@@ -51,7 +51,7 @@ public class CreateAssetApi private constructor(
         private const val DEFAULT_FILENAME = "unknown-file"
         private const val ASSET_API_FORM_PROPERTY_NAME = "file"
         private val HEADERS = Headers.build {
-            append(HttpHeaders.ContentType, "form-data")
+            append(HttpHeaders.ContentType, ContentType.MultiPart.FormData)
         }
 
         /**