Should raise exception if trying to use digest auth with net_http adapter

[leighhalliday]

Since digest auth isn't available to the net_http adapter, it should raise an exception instead of just ignoring it. Many hours of trouble and heartache because I didn't know why my digest auth wasn't working, when if a simple exception was raised, I would have solved it quickly.

[rogerleite]

@leighhalliday sorry about this! I'm gonna review authentication from all adapters.

[rubiii]

finally fixed on master. thanks @leighhalliday.