Update indirect rate in set_rate method

Hi! I've found some problem with `set_rate` method. 

For example, I have:
```ruby
@bank.flush_rates
@bank.add_rate('RUB', 'USD', 10.0)
```

But after that when I update indirect rate:
```ruby
@bank.add_rate('USD', 'RUB', 0.5)

# expect
@bank.get_rate('RUB', 'USD') # => (1.0 / 0.5) = 2.0
# but still have
@bank.get_rate('RUB', 'USD') # => 10.0
```

To fix it maybe it should update indirect rate too while setting direct rate in method `set_rate`.