customFilterListOptions.update ignores return values

customFilterListOptions.update ignores return values. In editors where you receive warnings about reassign to parameters (disable with // eslint-disable-next-line no-param-reassign ), one might be attempted to make a copy of filterList and return the copy, but this won't work.

## Expected Behavior

returning a deep copy of filterList should work.

## Current Behavior
This line of code
https://github.com/gregnb/mui-datatables/blob/a382f3e8e850c7fcdd395f8594a5e0afefcaf905/src/MUIDataTable.js#L1389

assigns to function parameter (unsafe per eslint rules)

When the function exits changes are lost.

Further, line 1403, does not accept changes to filterList if there had been any:
https://github.com/gregnb/mui-datatables/blob/a382f3e8e850c7fcdd395f8594a5e0afefcaf905/src/MUIDataTable.js#L1403

Also, this line of code in the example is meaningless (even though the function works correctly because it assigns directly to array elements by reference)
https://github.com/gregnb/mui-datatables/blob/ae1d49a61ddafd2a83ac445f279a0a67c434e3de/examples/customize-filter/index.js#L132
## Steps to Reproduce (for bugs)

1.
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## Your Environment

| Tech | Version |
|--------------|---------|
| Material-UI | |
| MUI-datatables | |
| React | |
| browser | |
| etc | |

Start in src/MUIDataTable.js around lines 1389 and 1403, then review examples/customize-filter/index.js around line 132. Trace how customFilterListOptions.update handles filterList and its return value. Done means returned deep-copy changes are preserved and the example no longer contains the meaningless line.

Written by the indexing model from the issue text.