Settings files with string keys clobbers settings instead of merging

[Fryguy]

If a config/settings.local.yml file is present, and the internal keys are strings, then it clobbers settings from other sections.

1. Create a config/settings.local.yml file such as

   prototype:
     test: test

2. Open a rails console

3. See that other settings are gone
   Before:

   > Settings.prototype.to_hash
   => {:amazon=>{:s3=>false}, :ems_vmware=>{:allow_direct_hosts=>false}, :ems_workflows=>{:enabled=>true}}

   After

   > Settings.prototype.to_hash
   => {:test=>"test"}

---

Note that if you use symbol keys the merging works fine

:prototype:
  :test: test

> Settings.prototype.to_hash
=> {:amazon=>{:s3=>false}, :ems_vmware=>{:allow_direct_hosts=>false}, :ems_workflows=>{:enabled=>true}, :test=>"test"}

---

I've created #23167 to at least give a clue that changes are present, since the settings.local.yml file is gitignored.

[Fryguy]

Note that if you modify non-local files (e.g. config/settings.yml) with string keys this also happens. I've changed the Issue title to reflect that it's really any settings files.