Make node handle NameError from host_inventory

[nownabe]

I found two errors of node.

First one is as follows.
It is found with node["undefined"]:

/Users/nownabe/itamae-test/vendor/bundle/ruby/2.2.0/gems/specinfra-2.12.1/lib/specinfra/host_inventory.rb:22:in `const_get': uninitialized constant Undefined (NameError)
    from /Users/nownabe/itamae-test/vendor/bundle/ruby/2.2.0/gems/specinfra-2.12.1/lib/specinfra/host_inventory.rb:22:in `[]'
    from /Users/nownabe/itamae-test/vendor/bundle/ruby/2.2.0/gems/itamae-1.1.3/lib/itamae/node.rb:19:in `[]'
    from /Users/nownabe/itamae-test/recipe.rb:1:in `initialize'

Second one is as follows.
It is found with node["un-defined"]:

/Users/nownabe/itamae-test/vendor/bundle/ruby/2.2.0/gems/specinfra-2.12.1/lib/specinfra/host_inventory.rb:22:in `const_get': wrong constant name Un-defined (NameError)
    from /Users/nownabe/itamae-test/vendor/bundle/ruby/2.2.0/gems/specinfra-2.12.1/lib/specinfra/host_inventory.rb:22:in `[]'
    from /Users/nownabe/itamae-test/vendor/bundle/ruby/2.2.0/gems/itamae-1.1.3/lib/itamae/node.rb:19:in `[]'
    from /Users/nownabe/itamae-test/recipe.rb:1:in `initialize'

[ryotarai]

Thank you for your contribution!
I also think #[] with unset key should return nil and I'll merge this.

@mizzy I think Specinfra's HostInventry#[] should return nil instead of raising NameError or NotImplementedError when the key is not found, like Hash#[]. How about you?

[ryotarai]

This is released in v1.1.4.

[mizzy]

@mizzy I think Specinfra's HostInventry#[] should return nil instead of raising NameError or NotImplementedError when the key is not found, like Hash#[]. How about you?

You're right. I will fix it later. Thanks!