Fix errors with encrypted subdoc arrays on Mongoose v6

As of Mongoose v6, encrypted subdocument arrays lead to the following
error when updating an existing document:

> MongoServerError: Updating the path '…' would create a conflict at '…'

This change circumvents the problem by marking the corresponding schema
paths as unmodified (which should be safe, since those encrypted fields
are not meant to be stored anyway).

Fixes joegoldbeck#105

lib/plugins/mongoose-encryption.js

@@ -345,6 +345,10 @@ var mongooseEncryption = function(schema, options) {
             // remove encrypted fields from cleartext
             encryptedFields.forEach(function(field){
                 setFieldValue(that, field, undefined);
+                // prevent conflict errors due to array change tracking in Mongoose 6
+                if (that.schema.paths[field].$isMongooseArray) {
+                    that.unmarkModified(field);
+                }
             });
 
             cb(null);

test/encrypt.coffee

@@ -1128,6 +1128,48 @@ describe 'Array EmbeddedDocument', ->
               done()
         return
 
+  describe 'when entire children array is encrypted', ->
+    before ->
+      ChildModelSchema = mongoose.Schema
+        text: type: String
+
+      ParentModelSchema = mongoose.Schema
+        children: [ChildModelSchema]
+
+      ParentModelSchema.plugin encrypt,
+        encryptionKey: encryptionKey,
+        signingKey: signingKey
+        encryptedFields: ['children']
+
+      @ParentModel = mongoose.model 'EncChildrenArrayParent', ParentModelSchema
+      @ChildModel = mongoose.model 'EncChildrenArrayChild', ChildModelSchema
+
+    beforeEach (done) ->
+      @parentDoc = new @ParentModel
+
+      childDoc = new @ChildModel
+        text: 'Child unencrypted text'
+
+      @parentDoc.children.addToSet childDoc
+
+      @parentDoc.save done
+
+    after (done) ->
+      @parentDoc.remove done
+
+    describe 'document.save()', ->
+      it 'should save changes to encrypted children', (done) ->
+        @ParentModel.findById @parentDoc._id, (err, doc) =>
+          assert.equal err, null
+          doc.children[0].text = 'New unencrypted text'
+          doc.save (err) =>
+            assert.equal err, null
+            @ParentModel.findById doc._id, (err, updatedDoc) ->
+              assert.equal err, null
+              assert.equal updatedDoc.children[0].text, 'New unencrypted text'
+              done()
+        return
+
   describe 'when child and parent are encrypted', ->
     before ->
       ChildModelSchema = mongoose.Schema