Copy kwargs dictionary

This fixes a bug where the client library makes changes to the dictionary sent by the API consumer, so that if you were to make two consecutive calls in a row, like this:

```
jobs_data = { ... }

# Post over our two jobs, use the same translator for both, don't pay for them
print gengo.determineTranslationCost(jobs=jobs_data)

# Post over our two jobs, use the same translator for both, don't pay for them
print gengo.postTranslationJobs(jobs=jobs_data)
```

The second call will not have the `as_group` parameter as part of the `jobs_data` dictionary anymore, because it was popped from the dictionary by this client library.