Implement default gem info to use when there is no depending gem is defined.
Merge pull request #2108 from take-cheeze/default_gem_info

Implement default gem info to use when there is no depending gem loaded.
Fix MSVC warning for gc.c
Merge pull request #2111 from kyab/fix_msvc_warning_gc.c

Fix MSVC warning for gc.c
Update documentation of `MRuby::Gem::Specification#add_dependency`.
Merge pull request #2113 from take-cheeze/add_dependency_doc

Update documentation of `MRuby::Gem::Specification#add_dependency`.
Array#fill fix bug when argument use range object
Array#fill use `kind_of?` instead of `respond_to?`