Schema deserializes map nat-nat as MichelsonMap<string, BigNumber> instead of MichelsonMap<BigNumber, BigNumber>

[psmolinsky]

Description

The class Schema deserializes Michelson corresponding to a map with nat key and nat value as Michelson<string, BigNumber>. However, nat is usually deserialized to BigNumber so it should be Michelson<BigNumber, BigNumber>.

Steps To Reproduce
Steps to reproduce the behavior:

1. Run this code:

import { Schema } from '@taquito/michelson-encoder';

const schema = new Schema({
    prim: 'map',
    args: [
        { prim: 'nat' },
        { prim: 'nat' },
    ],
});
const michelson = [{
    prim: 'Elt',
    args: [
        { int: '0' },
        { int: '1000000000000' }
    ]
}];

const value = schema.Execute(michelson);
const entry = Array.from(value.entries())[0];

console.log('Entry:', entry);
console.log('Key type:', typeof entry[0]);

2. It output on console:

Entry: [ '123', BigNumber { s: 1, e: 2, c: [ 456 ] } ]
Key type: string

Expected behavior

It should output:

Entry: [ BigNumber { s: 1, e: 2, c: [ 123 ] }, BigNumber { s: 1, e: 2, c: [ 456 ] } ]
Key type: object