!!str and quotes ignored when loading merge keys

[connec]

a: &me { hello: world }
b: { !!str '<<': *m }

This loads as:

{
  "a": { "hello": "world" },
  "b": { "hello": "world" }
}

It should load as:

{
  "a": { "hello": "world" },
  "b": {
    "<<": { "hello": "world" }
  }
}

[connec]

This causes issues when dumping hashes with a "<<" key and an alias:

h = {}
Psych.dump 'a' => h, 'b' => h
# => "---\na: &1 {}\nb: *1\n"
Psych.dump 'a' => h, '<<' => h
# => "---\na: &1 {}\n\"<<\": *1\n"
Psych.load _
# => {"a"=>{}}

This is pretty 'edge-casey' and unlikely to come up in real code, but it has some interesting implications for #202.

[tenderlove]

I think if we fix this, then dealing with #202 becomes much easier.