Give upgraded strings to SQLite as downgraded when sqlite_unicode==0.

Issue #66: If $a and $b contain the same code points but one is upgraded
and the other isn’t, previously DBD::SQLite would give two different C
strings to SQLite. This changeset fixes that so that SQLite always
translates equivalent Perl strings to the same C string, regardless of the
Perl strings’ internal encoding.

dbdimp.c

@@ -549,6 +549,9 @@ sqlite_db_do_sv(SV *dbh, imp_dbh_t *imp_dbh, SV *sv_statement)
     if (imp_dbh->unicode) {
         sv_utf8_upgrade(sv_statement);
     }
+    else {
+        sv_utf8_downgrade(sv_statement, 0);
+    }
 
     statement = SvPV_nolen(sv_statement);
 
@@ -893,6 +896,9 @@ sqlite_st_prepare_sv(SV *sth, imp_sth_t *imp_sth, SV *sv_statement, SV *attribs)
     if (imp_dbh->unicode) {
         sv_utf8_upgrade(sv_statement);
     }
+    else {
+        sv_utf8_downgrade(sv_statement, 0);
+    }
 
     statement = SvPV_nolen(sv_statement);
 
@@ -1007,9 +1013,11 @@ sqlite_st_execute(SV *sth, imp_sth_t *imp_sth)
             int numtype = 0;
 
             if (imp_dbh->unicode) {
-                sv_utf8_upgrade(value);
+                data = SvPVutf8(value, len);
+            }
+            else {
+                data = SvPVbyte(value, len);
             }
-            data = SvPV(value, len);
 
             /*
              *  XXX: For backward compatibility, it'd be better to

t/68_upgraded_no_unicode.t

@@ -0,0 +1,61 @@
+# This is a test for correct handling of upgraded strings without
+# the sqlite_unicode parameter.
+
+use strict;
+use warnings;
+use lib "t/lib";
+use SQLiteTest;
+use Test::More;
+use if -d ".git", "Test::FailWarnings";
+
+{
+    my $dbh = connect_ok( dbfile => 'foo', RaiseError => 1 );
+
+    my $tbl_name = "\xe9p\xe9e";
+    utf8::encode $tbl_name;
+
+    my $str = "CREATE TABLE $tbl_name ( col1 TEXT )";
+    utf8::upgrade $str;
+
+    $dbh->do($str);
+
+    my $master_ar = $dbh->selectall_arrayref('SELECT * FROM sqlite_master', { Slice => {} });
+
+    is(
+        $master_ar->[0]{'name'},
+        $tbl_name,
+        'do() takes correct string value',
+    );
+
+    #----------------------------------------------------------------------
+
+    my $dummy_str = "SELECT '$tbl_name'";
+    utf8::upgrade $dummy_str;
+
+    my $sth = $dbh->prepare($dummy_str);
+    $sth->execute();
+    my $row = $sth->fetchrow_arrayref();
+
+    is(
+        $row->[0],
+        $tbl_name,
+        'prepare() takes correct string value',
+    );
+
+    #----------------------------------------------------------------------
+
+    my $tbl_name_ug = $tbl_name;
+    utf8::upgrade $tbl_name_ug;
+
+    my $sth2 = $dbh->prepare('SELECT ?');
+    $sth2->execute( do { my $v = $tbl_name_ug } );
+    $row = $sth2->fetchrow_arrayref();
+
+    is(
+        $row->[0],
+        $tbl_name,
+        'execute() takes correct string value',
+    );
+}
+
+done_testing;