test: Adds tests for failing header case

Author: thecodedrift

Diff for: src/client.ts

@@ -626,11 +626,14 @@ export class Collection<TSchema = Document> {
         errorText = ((await response.clone().json()) as { error: string })
           ?.error;
       } catch {
+        /* c8 ignore start */
+        // used to ensure maximum chance of retieving data from the Realm API
         try {
           errorText = await response.clone().text();
         } catch {
           errorText = undefined;
         }
+        /* c8 ignore stop */
       }
 
       const fallbackMessage = {

Diff for: test/mocks/handlers.ts

@@ -142,6 +142,11 @@ export const handlers = [
       return response(ctx.status(400), ctx.json(errors["400"]));
     }
 
+    // missing ejson header is a hard 400 error
+    if (!request.headers.get("content-type")?.includes("application/ejson")) {
+      return response(ctx.status(400), ctx.json(errors["400"]));
+    }
+
     // incorrect auth results in a 401 from mongo
     let auth = false;
     if (request.headers.get("authorization") === `Bearer ${validJWT}`) {