Treat objects that do not implement the `as_sql` method as bind parameters

This pull request fixes an incompatibility issue introduced in #35.

Prior to the merger of #35, blessed objects were treated as bind parameters.  But #35 changed the behavior; all blessed objects were handled as if they were query objects that implement `as_sql` and `bind` methods.

This pull requests fixes the problem by adding checks before calling the `as_sql` method, if the blessed object does not implement the method, it is handled as a bind parameter so that the database driver would actually stringify the object.

See also: https://twitter.com/hisaichi5518/status/477385176830337024