Typo in EIP721Base domain separator implementation

[ivandiazwm]

It looks like there's a typo in EIP712Base implementation. It called getDomainSeperator but it should be getDomainSeparator
https://github.com/maticnetwork/pos-portal/blob/master/contracts/common/EIP712Base.sol#L46
It looks like this issue is repeated in variables and other functions.

I can open a PR but I'm not sure if it's ok since it would change the contract's interface.
DAI proxy's implementation contract depends on this getDomainSeperator function and it's deployed with the typo.