Preserve valid identifiers in SQL-DF import

[snunez1]

Discussed in Lisp-Stat/lisp-stat#24

^{Originally posted by gassechen September 27, 2023}
Hello, I have a question.
If I use pprint to print the data frame these are the results

(pprint df2)
;; ESN       ASSET_ID          
;; 0-4513034   672061 
;; 2-3211115   609473 

but if I use them with sqldf they are these

(pprint (sqldf:sqldf  "select esn,ASSET_ID from DF2 limit 10"))
;; ESN       ASSET-ID
;; 0-4513034   672061
;; 2-3211115   609473

It is changing the names of the keys
ASSET_ID --> ASSET-ID

SQL-DF permanently modifies identifiers into idiomatic lisp format, even those that are valid lisp identifiers, but non-idiomatic. This should be configurable.