Fixed concurrent modification bug in hash iterator for net_server_args

[kriegfrj]

I was alerted to this by warnings on my console during testing.

The code in Plack::Handler::Starman that implemented #111 was attempting to modify the $self hash (adding the net_server_args entry) while iterating over it using each. This causes the iterator to become undefined because the insertion of a new element can change the iterator order. Fixed by accumulating all the options in a sub-hash first, and then adding the sub-hash (if non-empty) when the iteration is complete.

[miyagawa]

Good catch!

[kriegfrj]

At least all my messing around in #112 bore some fruit. :wink: