GetAssetsRequest.AssetClass should be type AssetClass (instead of string)

[gcatlin]

There is already an AssetClass type, so it's just a matter of using it.

That is, change this:

type GetAssetsRequest struct {
	Status     string
	AssetClass string
	Exchange   string
}

To this:

type GetAssetsRequest struct {
	Status     string
	AssetClass AssetClass
	Exchange   string
}