Instantiate a section with default values even for multi: true sections

[tagomoris]

Previously, "init: true, multi: false" sections were not instantiated (just [] for instance variable).
But with this behavior, we cannot use default values of these sections, except for the case to set a
blank Config::Element into conf.elements.
It looks abnormal, and non-useful feature for plugin authors.

So, this change will improve the situation.

Currently, there are some example without this fix:

config_section :parse do
  config_set_default :@type, "yay"
end

def configure(conf)
  super
  # ...
  @parser = parser_create(conf: conf.elements('parse').first, default_type: "yay")
end

This code has default type specification twice: config_set_default and default_type.
With this change:

config_section :parse do
  config_set_default :@type, "yay"
end

def configure(conf)
  super
  # ...
  @parser = parser_create(conf: conf.elements('parse').first)
end

Furthermore, this code works as you expected (for <parse> section without any argument).

config_section :parse do
  config_set_default :@type, "yay"
end

def configure(conf)
  super
  @parser = parser_create # default usage(argument) is ''
end

This looks better than ever.

[tagomoris]

@repeatedly How do you think about this change?

[repeatedly]

LGTM.