This fixes a PropType error when using non-custom slippage, and it
fixes a type inconsistency when custom slippage is used.
Previously, `slippage` was being converted explicitly to a `Number` as
it was passed into `BuildQuote`, but not as it was passed into
`AwaitingSwap`. Also the PropType was set as `string`, despite the fact
that it's a number in most cases, and is used for math.
The PropType has been changed to `number`, and the selective casting to
`Number` has been removed. Instead, the `maxSlippage` value is cast to
a `Number` as it's being selected, so that the type is always
consistent.
David Walsh
Mark Stacey
Brad Decker
Dan J Miller