python client `df.drop` call changes input parameter

Following code using default credit card dataset:

```
import h2o

h2o.init(port=13334)
df = h2o.import_file("CreditCard_Cat-train.csv")

dropped = ["PAY_0"]
print(dropped)

df.drop(dropped)
print(dropped)
```

has following output:
```
['PAY_0']
[-7]
```

which shows that the variable `dropped` has change after the `df.drop` call from `['PAY_0']` to `[-7]`.

This means that user can not call `drop` on multiple frames in a row with the same input variable.