Columns with upper-case letters break not_null tests

[dividor]

Describe the bug
Seems DB columns with upper case letters need to be quoted in postgres. DBT doesn't account for this (our version anyway) so not_null tests fail

To Reproduce
Steps to reproduce the behavior:

1. Change one of the demo data columns to have an upper-case leter in its name
2. Create a not_null test for this column
3. Run DOT
4. DOT will fail

Expected behavior
Should support columns with capital letters