better metadata reading to tokens from being stuck in the bridge

[josojo]

closes #222

[edmundedgar]

Is there a reason for our fallbacks to be named subtly differently from the way upstream does it?

https://github.com/0xPolygonHermez/zkevm-contracts/blob/d70266b8742672d59d4060019538d03fe0aac181/contracts/PolygonZkEVMBridge.sol#L800

[edmundedgar]

Code looks fine, except the comment that polygon upstream seems to be subtly different, it would be good to align it if there isn't a reason not to.

Also I would suggest either adding tests or making an issue or a TODO comment (either in the original code or in the place where the tests should go) that we should add tests.

[josojo]

I reused the functions from the bridge contract. It's kinda ucly code duplications, but I can not think of a better method, since I dont' wanna modify the zkevm-contracts repo and refactor the changes there, to keep the diff minimal to the polygon zkevm

[edmundedgar]

Awesome, looks great.

I wouldn't worry about the fact that this copy-pastes some code, I think it's better to copy-paste than do something ugly to make a dependency work.