Store report cache using buffer

SQLite doesn't detect the binary even if stored in BLOB if the type is not
buffer.

issue2704
review1941002
(transplanted from 3b77f743131cb946fbecd50bc49cb418c67d584e)

invoice.py

@@ -2209,7 +2209,7 @@ class InvoiceReport(Report):
         if invoice.state in ('open', 'paid'):
             invoice_obj.write(invoice.id, {
                 'invoice_report_format': res[0],
-                'invoice_report_cache': res[1],
+                'invoice_report_cache': buffer(res[1]),
                 })
         return res