sql.py: cast(): Removed conditional checks for save_errors, since it's now always true if the function got passed the `not save_errors` special case

sql.py: cast(): Only convert errors to warnings if errors will be saved in errors_table, so that import will always be aborted if user supplied invalid values in the mappings, even if these values are passed through a relational function

sql.py: put_table(): Support inserting tables with all default values, by providing the pkey's default value for all rows so that the SELECT query has at least one column

sql_gen.py: is_table_col(): Check that input is a Col object

sql.py: put_table(): Assert that mapping is non-empty

sql.py: mk_select(): Assert that fields list is non-empty

sql.py: DbConn.DbCursor.execute(): Set _is_insert only if query starts with INSERT, so that function definitions containing INSERT are not cached as INSERT statements (exceptions only) themselves

sql.py: DbConn.DbCursor.execute(): Fixed bug where params == None would apparently turn off the mogrifier completely, causing "%"s to be excessively escaped, by just setting params to None if it was [] or () and not using strings.esc_for_mogrify() at all

sql.py: DbConn.DbCursor.execute(): If not using params, escape the query using strings.esc_for_mogrify() in case any literals contained "%"s

strings.py: Added esc_for_mogrify()