Don't stringify JSON again in fetchUrlResponse

fetchUrlResponse is called from queryApiWithFallback and is given stringified
JSON data, but then stringifies the string again. This leads to errors when
trying to view source files with a local samply server.

Author: eqrion

src/utils/query-api.ts

@@ -137,7 +137,7 @@ export class RegularExternalCommunicationDelegate
     const requestInit: RequestInit =
       postData !== undefined
         ? {
-            body: JSON.stringify(postData),
+            body: postData,
             method: 'POST',
             mode: 'cors',
             credentials: 'omit',