SQL: add warning for mismatch in provided and written table name (GH7815)
Closes #7815

Adds a warning if the provided table name in `to_sql` and the actually used table name in the database do not match (eg converted to lower case due to case insensitivity settings of database, eg MySQL on Windows). When user is not aware of this, and keeps using the upper case table name, this leads to bugs in subsequent `to_sql` calls.

@maxgrenderjones 
