fix Point.prototype.toJSON for short curves

[mvayngrib]

thanks @fanatid for helping me look into this

[fanatid]

Short explanation:

p192, p224, curve25519, ed25519, secp256k1 use Base reduction engine
p256, p384, p521 use Montgomery method engine

BN-Red for curve created at https://github.com/indutny/elliptic/blob/v6.5.0/lib/elliptic/curve/base.js#L14

On serialization BN#toJSON called without checking red existence: https://github.com/indutny/bn.js/blob/v5.0.0/lib/bn.js#L529-L531

While Base reduction engine call just remove red property: https://github.com/indutny/bn.js/blob/v5.0.0/lib/bn.js#L3386-L3390

Montgomery method engine makes manipulations: https://github.com/indutny/bn.js/blob/v5.0.0/lib/bn.js#L3422-L3426

As result if point with Montgomery method will be serialized result will not be same on deserialization.

[jprichardson]

@indutny @fanatid can we get this merged? Anything more we can help with to move this forward?

[fanatid]

LGTM