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 = { ... }

# Get a quote for the jobs
print gengo.determineTranslationCost(jobs=jobs_data)

# Request the jobs
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.