Prevent http client from converting our form data (#56772) (#56792)

If we don't specify an undefined content-type like so, the client
attempts to convert our form data into a JSON string, and the backend
returns an understandable 415.

This fixes rule imports.

Co-authored-by: Elastic Machine <[email protected]>

Author: rylnd

x-pack/legacy/plugins/siem/public/containers/detection_engine/rules/api.ts

@@ -246,6 +246,7 @@ export const importRules = async ({
     `${DETECTION_ENGINE_RULES_URL}/_import`,
     {
       method: 'POST',
+      headers: { 'Content-Type': undefined },
       query: { overwrite },
       body: formData,
       asResponse: true,