add "rds_csv_quote_string" command

[lloydzhou]

can add one command to control this module to quote the string field or not.
so if using this command and command "rds_csv_field_name_header", we can server binary data from RDS upstream (eg: image).

location ~ '/cats/image/([0-9]+)' {
    drizzle_query 'select img from cat_image where id=$1 limit 1';
    drizzle_pass backend;
    rds_csv on;
    rds_csv_quote_string off;
    rds_csv_field_name_header off;
    rds_csv_content_type 'image/jpeg';
}

i have write simple code on the fork lloydzhou@5c26493.

[agentzh]

@lloydzhou Thank you for your contribution! Will you create a proper github pull request with your commit? See https://help.github.com/articles/creating-a-pull-request Thanks!