sendTransaction() should accept only 1 private key

[deeintech]

Description

Sending a transaction using sendTransaction function by the omg-js library currently takes an array of private keys causing Uncaught (in promise) Error: Number of non-empty inputs should match signatures count. Remove redundant signatures issue. This happens because a function doesn't know how many UTXOs it will use since it is calculated internally.

Changing the fromPrivateKeys property from an array into a single private key should make things work.