Back office asset invoice date filter fails in a different language

### Steps to reproduce

Go to top right corner -> my profile -> preferences and set your language to "polish"
Go to back office -> hardware
Find an asset with a certain invoice date, let's say 5/11/2018
Select a range in the filter on the left from 4/11/2018 to 6/11/2018
Press the "filter" button.

### Expected behavior

The system shows records that match the date.

### Actual behavior

The system shows all the records just as if there was no filter set.

### Environment

* Ralph version: 20190111.1
* Operating system: ubuntu 18.04 minimal/server on a VM, architecture: amd64
* Method of installation: Deb package

### Additional info:

The same search on the same user, but with language preference set to "english" returns only assets from the selected range, just as expected.

Notable differences:

Changing the language changes how dates are displayed in asset records:
In english:
**Nov. 5, 2018**
In polish:
**5 listopada 2018**

Filter displays the date after picking it:
In english:
2018-11-04 2018-11-06
In polish:
2018-11-04 2018-11-06

editing an asset, after picking a date:
In polish:
it displays as 2018-11-05, and after clicking "Zapisz"(Save) it displays as **05.11.2018**
In english:
it displays as 2018-11-05, and after clicking "Save" it displays as **2018-11-05**

Differences highlighted in bold. It might be worth to check those, as the bug probably stems from date conversion from polish to english format somewhere along the way.