Include MKR token edge case in _getBestAPR in APROracle.sol

[deepcrazy]

• _getBestAPR is handling the edge for WETH token as of now. However, if using getTokenConfigBySymbol(symbol) of Compound, then MKR token's edge case can be added as MKR contract returns MKR's symbol's hash and not "MKR" as string.
• This has to be explored bit more as well as if MKR is possible scenario or not.
• Also, Maybe we can use getTokenConfigByUnderlying(token) with an exception for MKR. However, this function also returns error right now for MKR token address and therefore needs more exploration.