Fix bug in xform_name for already transformed names

This is a regression introduced from https://github.com/boto/botocore/pull/240

When you try to create a trail you'll get an error:

```
Unknown parameter 's_3_bucket_name' for operation Operation:CreateTrail.  Must b
e one of: name, s3_bucket_name, s3_key_prefix, sns_topic_name, include_global_se
rvice_events, trail
```

The issue is that we xform `s3_bucket_name` again and get `s_3_bucket_name`.  The fix here is to not translate names if they already appear to be translated.