BUG: Fix inconsistent C engine quoting behaviour
1) Add significant testing to quoting in `read_csv`

2) Fix bug in C engine in which a NULL `quotechar` would raise even though `quoting=csv.QUOTE_NONE`.

3) Fix bug in C engine in which `quoting=csv.QUOTE_NONNUMERIC` wouldn't cause non-quoted fields to be cast to `float`.  Relevant definitions can be found in the Python docs <a href="https://docs.python.org/3.5/library/csv.html">here</a>.
