Beneficiary is credited additional ETH above premium

[HardlyDifficult]

From horsefacts in #294

Beneficiary is credited additional ETH above premium
The Cally#buyOption function ensures that the caller sends an ETH amount equal to or greater than the calculated premium:

buyOption#L224

   require(msg.value >= premium, "Incorrect ETH amount sent");

It then credits the beneficiary with an amount equal to msg.value:

buyOption#L250

    ethBalance[beneficiary] += msg.value;

If the caller of buyOption sends excess ETH above the premium amount, this additional amount is credited to the beneficiary.

Recommendation: If this is intentional, clearly document this behavior for end users. If not, consider requiring an exact premium amount rather than accepting additional ETH.

[HardlyDifficult]

Dupe of #84

[JeeberC4]

Issue recreated with script that includes all required data.