Allow saving to xml over an existing sqlite3 file and the other way around

A type check on the pre-existing file was preventing this. The type check however
only makes sense when opening files, not when overwriting.

libgnucash/engine/qofsession.cpp

@@ -180,7 +180,9 @@ QofSessionImpl::load_backend (std::string access_method) noexcept
             continue;
         }
         PINFO (" Selected provider %s", prov->provider_name);
-        if (!prov->type_check (m_book_id.c_str ()))
+        // Only do a type check when trying to open an existing file
+        // When saving over an existing file the contents of the original file don't matter
+        if (!m_creating && !prov->type_check (m_book_id.c_str ()))
         {
             PINFO("Provider, %s, reported not being usable for book, %s.",
                     prov->provider_name, m_book_id.c_str ());
@@ -303,6 +305,7 @@ QofSessionImpl::begin (std::string new_book_id, bool ignore_lock,
     destroy_backend ();
     /* Store the session URL  */
     m_book_id = new_book_id;
+    m_creating = create;
     if (filename)
         load_backend ("file");
     else                       /* access method found, load appropriate backend */
@@ -481,7 +484,7 @@ QofSessionImpl::save (QofPercentageFunc percentage_func) noexcept
     }
     else
     {
-        push_error (ERR_BACKEND_NO_HANDLER, "failod to load backend");
+        push_error (ERR_BACKEND_NO_HANDLER, "failed to load backend");
         LEAVE("error -- No backend!");
     }
     m_saving = false;

libgnucash/engine/qofsession.hpp

@@ -95,6 +95,7 @@ private:
     std::string m_book_id;
 
     bool m_saving;
+    bool m_creating;
 
     /* If any book subroutine failed, this records the failure reason
      * (file not found, etc).