v.db.update: Use case insensitive comparison for columns (#1413)

Returned column names are with case as present in the table, but since unqoted column names in SQL are case insensitive,
the constructed UPDATE statement works in any case.

Without this PR, the following fails for column called ABC:
...column=abc where='abc IS NULL'...
although the where part as well as rest of the UPDATE statement would work.

scripts/v.db.update/v.db.update.py

@@ -92,10 +92,14 @@ def main():
         if not os.access(sqlitefile, os.R_OK):
             grass.fatal(_("File <%s> not found") % sqlitefile)
 
-    # checking column types
-    try:
-        coltype = grass.vector_columns(vector, layer)[column]["type"]
-    except KeyError:
+    # Check column existence and get its type.
+    all_columns = grass.vector_columns(vector, layer)
+    coltype = None
+    for column_name, column_record in all_columns.items():
+        if column.lower() == column_name.lower():
+            coltype = column_record["type"]
+            break
+    if not coltype:
         grass.fatal(_("Column <%s> not found") % column)
 
     if qcolumn: