Support mock value for fields without setter

[nykolaslima]

Example:

public class Account {
private BigDecimal balance;
public BigDecimal getBalance() { return balance; }
// No Setter for balance
}

Fixture-factory should be able to mock "balance" field value. Nowadays this throw an IllegalArgumentException with no such attribute message.

Fixture.of(Account.class).addTemplate("balance", new Rule(){{
add("balance", new BigDecimal("1000");
}});