test coverage now is 97%, fixed drop_extension method for postgres adapter

Task 13294 - improve test coverage to 97%

### Changes
* Added more spec
* Fixed `AVAILABLE_DROP_MODES.keys` in `drop_extension` method in lib/pg_power/connection_adapters/postgresql_adapter/extension_methods.rb

### RSpec
```
Finished in 0.54852 seconds
153 examples, 0 failures, 3 pending
```
Now coverage is 97.06%

### Doc
Not affected.