Multiselect EAV Product Attribute not working in Mass Action

## Steps to reproduce
1. Install Magento 2.1
2. Create a product attribute with frontend input "multiselect" (and assign it to the necessary attribute sets).
## Expected result
1. You can set values via mass action.
## Actual result
1. If you want to use it via mass action from the grid, it does not set the selected values on the product(s).
