More DB -> CSV fixes and some additions

[cpcloud]

• pass encoding to sniffer
• test that we can handle headers with quoted field names
• use transactions with begin when writing to csv
• Output CSV files always have a header unless we're using MySQL, so change that attr when returning the CSV instance.
• test that we can handle outputting NULL values to CSV from postgres
• that that we can read in files with a different encoding

[cpcloud]

passing on windows. merging