Restore state option mergeArrays: false

The types had the incorrect default so I flipped this the wrong way previously. See: hapijs/hoek#395

src/server/plugins/engine/pageControllers/PageControllerBase.ts

@@ -510,11 +510,7 @@ export class PageControllerBase {
   /**
    * deals with parsing errors and saving answers to state
    */
-  async handlePostRequest(
-    request: Request,
-    h: ResponseToolkit,
-    mergeOptions?: merge.Options
-  ) {
+  async handlePostRequest(request: Request, h: ResponseToolkit) {
     const { cacheService } = request.services([])
     const payload = (request.payload || {}) as FormPayload
     const formResult = this.validateForm(payload)
@@ -553,7 +549,7 @@ export class PageControllerBase {
       return
     }
 
-    await cacheService.mergeState(request, update, mergeOptions)
+    await cacheService.mergeState(request, update)
   }
 
   /**

src/server/services/cacheService.ts

@@ -29,16 +29,14 @@ export class CacheService {
     return cached || {}
   }
 
-  async mergeState(
-    request: Request,
-    value: object,
-    mergeOptions?: merge.Options
-  ) {
+  async mergeState(request: Request, value: object) {
     const key = this.Key(request)
     const state = await this.getState(request)
     const ttl = config.get('sessionTimeout')
 
-    merge(state, value, mergeOptions)
+    merge(state, value, {
+      mergeArrays: false
+    })
 
     await this.cache.set(key, state, ttl)
     return this.cache.get(key)