LWT: Inserting Subset of columns returns all columns

See: https://github.com/gocql/gocql/issues/792#issuecomment-248983669

When inserting a subset of the table columns with the use of light weight transactions the cassandra result returns a full set of unordered cassandra column values.

SETUP:
```
CREATE TABLE IF NOT EXISTS test.inserttest(
key bigint,
session_token text,

foo text,
bar text,
PRIMARY KEY(key, event_date, session_token);

INSERT INTO test.inserttest(key, session_token, foo) VALUES (1, 'myToken', 'baz') IF NOT EXISTS;
```

`insert into test.inserttest(key, session_token, foo) VALUES (1, 'myToken', 'bez') IF NOT EXISTS;`

Expected result: Returns False, 1, myToken, baz
Actual result: Returns true and all column values.