Only overwrite pass_block if there's a value

This prevents pass_block from being overwritten with nil which happens when the superclass has (non-matching) routes.

This bites in development as Sinatra::Base registers "get '/__sinatra__/:image.png'" only in development mode, so everything works fine in production but not in development.
Merge pull request #708 from rlivsey/pass_block-with-superclass

Only overwrite pass_block if there's a value