Coin.IsGTE bad logic

[rigelrozanski]

https://github.com/tendermint/basecoin/blob/master/types/coin.go#L96-L102

assumptions

• IsGTE means Greater Than Or Equal (>=)
• A wallet which contains only 0 of coin 'X' is equal to an empty wallet (AKA have zero of a coin is like not having any reference to that coin in your wallet)

Orig code:

func (coinsA Coins) IsGTE(coinsB Coins) bool {
	diff := coinsA.Minus(coinsB)
	if len(diff) == 0 {
		return true
	}
	return diff.IsPositive()
}

This function will incorrectly return false under the circumstance that coinA and coinB contain different coins but of an amount of zero. Under this circumstance diff != 0 because there are different coins, however diff.IsPositive() returns false because it's not positive it is zero. The fix is to simply replace diff.IsPositive() with diff.IsNonnegative()