[TokenRatesController] - Keep addresses in checksum format (#4377)

## Explanation

When adding market data for tokens in
#4206, it also changed the token
addresses to be lowercased instead of checksum. This PR moves it back to
the checksum format, since some places on the client expected it.

## References

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* Fixes #12345
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## Changelog

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### `@metamask/assets-controllers`

- **CHANGED**: Token rates controller uses checksum instead of lowercase
format for token addresses

## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've highlighted breaking changes using the "BREAKING" category
above as appropriate

packages/assets-controllers/src/TokenRatesController.ts

@@ -555,7 +555,7 @@ export class TokenRatesController extends StaticIntervalPollingControllerV1<
       (obj, [tokenAddress, token]) => {
         obj = {
           ...obj,
-          [tokenAddress.toLowerCase()]: { ...token },
+          [tokenAddress]: { ...token },
         };
 
         return obj;