Fuzzy matcher & Exact matcher

It would be nice if there is another matcher that does not check keys but does values.
(We have room for thoughts of the matcher names.)

```ruby
actual = { "a" => "b", "c" => "d" }.to_json

# same with the current `be_json` matcher
actual.should be_json_as("a" => "b", "c" => "d")

# almost same but allows subset of `actual`
actual.should be_json_including("a" => "b")
```