added assignment from value, conversion to database_type and appropriate tests

source/orm/sql/field.cpp

@@ -62,6 +62,10 @@ int field::index() const {
   return index_;
 }
 
+utils::value field::value() const {
+  return value_;
+}
+
 std::ostream &operator<<(std::ostream &out, const field &col) {
   out << col.str();
   return out;