ensure db is closed after block

avoid GC Warning

src/db/database.cr

@@ -22,6 +22,8 @@ module DB
     # Closes all connection to the database.
     def close
       @connection.try &.close
+      # prevent GC Warning: Finalization cycle involving discovered by mysql implementation
+      @connection = nil
     end
 
     # :nodoc:

src/db/db.cr

@@ -101,10 +101,13 @@ module DB
   # Same as `#open` but the database is yielded and closed automatically.
   def self.open(uri : URI | String, &block)
     build_database(uri).tap do |db|
+      begin
         yield db
+      ensure
         db.close
       end
     end
+  end
 
   private def self.build_database(connection_string : String)
     build_database(URI.parse(connection_string))