fix: restrict this to JSON, write the new content-length

It is possible that the content length is not equivalent due to JSON
serialization, so we need to re-write the Content-Length header to the
upstream.

This commit also restricts the fix to ONLY handle JSON. Other content
types could be handled with some conditionals if necessary.

src/http-proxy-middleware.ts

@@ -180,8 +180,10 @@ export class HttpProxyMiddleware {
   };
 
   private fixBody = (proxyReq: ClientRequest, req: Request) => {
-    if (req.body instanceof Object) {
-      proxyReq.write(JSON.stringify(req.body));
+    if (req.is('application/json')) {
+      const bodyData = JSON.stringify(req.body);
+      proxyReq.setHeader('Content-Length', Buffer.byteLength(bodyData));
+      proxyReq.write(bodyData);
     }
   };
 }