Fixed merge operator when both source and target are strings

closes #25

src/worker/execution-plugins/merger.spec.ts

@@ -22,6 +22,11 @@ describe('merger', () => {
       }
     },
     payload3: undefined,
+    payload4: {
+      nestedValues: {
+        one: 'whatever',
+      },
+    },
     valueNotToBeMerged: {
       iMust: 'be on result'
     }
@@ -84,4 +89,23 @@ describe('merger', () => {
       done()
     })
   })
+
+  it('should merge strings even with empty objects from next merges', (done) => {
+    const options = {
+      sourceFields: [
+        'payload.nestedValues.one',
+        'payload4.nestedValues.one',
+      ],
+      targetField: 'newBody.deep'
+    }
+
+    const objTransformer = new MergerPlugin(msg, {options}, '')
+
+    objTransformer.execute((err, mergedObj) => {
+      expect(err).to.be.null
+      expect(mergedObj.newBody.deep).to.be.an('string')
+      expect(mergedObj.newBody.deep).to.equal('whatever')
+      done()
+    })
+  })
 })

src/worker/execution-plugins/merger.ts

@@ -57,6 +57,10 @@ export default class MergerPlugin {
     })
 
     const mergedResult = slicedObjects.reduce((prevObj: any, currObj: any) => {
+      if (typeof prevObj === 'string' && typeof currObj === 'string') {
+        return currObj
+      }
+
       return merge(prevObj, currObj)
     }, [])