No css classes in header column in Production

Thead th cell has span inside it with role="button" and class="MUIDataTableHeadCell-toolButton-[mui-number-prefix]" in development mode. But in production there is no class="MUIDataTableHeadCell-toolButton-[mui-number-prefix]".

## Expected Behavior
The class must be present in production mode as well as in Development one.

## Current Behavior
The class is present just in development mode.

## Steps to Reproduce (for bugs)
1. Add table to the page.
2. Inspect a document in development and production mode.
3. See that class is not present in Production.

## Your Environment
| Tech | Version |
|--------------|---------|
| Material-UI | 4.11.0|
| MUI-datatables | 3.4.0|
| React | 16.13.1|
| browser | Chrome |

Attacments:
Dev:

Prod:

Reproduce the header column behavior in both development and production builds using React 16.13.1, Material-UI 4.11.0, and MUI-datatables 3.4.0. Trace how the header th span's role and class are generated and compare the resulting markup between builds. Done means the expected tool-button class is present in production as well as development.

Written by the indexing model from the issue text.