Method setOptions must honor the type of option param

[yriveiro]

The documentation of this method says that option param it's an array. We are converting internally the param option to an array if it's not. This can be done forcing the type of the param in the signature method.

The signature method:

public function setOptions($options) : BackoffInterface

should be:

public function setOptions(array $options) : BackoffInterface

This can break some scenarios, but this behaviour is wrong and should be corrected.