Improve key formatting for nested hashes and disable by default

[tf]

Since #486, key format is applied deeply to hashes and arrays that are
passed to set!, merge! and array!:

json.key_format! :upcase
json.set!(:foo, {some: "value"})

# => {"FOO": {"SOME": "value"}}

json.key_format! :upcase
json.merge!({some: "value"})

# => {"SOME": "value"}

json.key_format! :upcase
json.array!([{some: "value"}])

# => [{"SOME": "value"}]

This also works for arrays and hashes extracted from objects:

comment = Struct.new(:author).new({ first_name: 'John', last_name: 'Doe' })

json.key_format! camlize: :lower
json.set!(:comment, comment, :author)

# => {"comment": {"author": {"firstName": "John", "lastName": "Doe"}}}

This breaks code that relied on the previous behavior. Add a
deep_format_keys! directive that can be used to opt into this new
behavior and disable it by default.

 json.key_format! camelize: :lower
 json.deep_format_keys!
 json.settings({some_value: "abc"})

 # => { "settings": { "someValue": "abc" }}

Moreover, as a side effect of #486, key format was also applied to the
result of nested blocks, making it impossible to change key format in
the scope of a block:

json.key_format! camelize: :lower
json.level_one do
  json.key_format! :upcase
  json.value 'two'
end

# => jbuilder 2.10.0: {"levelOne": {"VALUE": "two"}}
# => jbuilder 2.11.0: {"levelOne": {"vALUE": "two"}}

The string "vALUE" results from calling
"value".upcase.camelize(:lower). The same happens when trying to
change the key format inside of an array! block.

This happens since key transformation was added in the _merge_values
method, which is used both by merge! but also when set! is called
with a block.

To restore the previous behavior, we pull the _transform_keys call
up into merge! itself. To make sure extracted hashes and arrays keep
being transformed (see comment/author example above), we apply
_transform_keys in _extract_hash_values and
_extract_method_values.

This also aligns the behavior of extract! which was left out in #486:

comment = {author: { first_name: 'John', last_name: 'Doe' }}

result = jbuild do |json|
  json.key_format! camelize: :lower
  json.extract! comment, :author
end

# => jbuilder 2.10 and 2.11: {"author": { :first_name => "John", :last_name => "Doe" }}
# => now: {"author": { "firstName": "John", "lastName": "Doe" }}

Finally, to fix array!, we make it call _merge_values directly
instead of relying on merge!. array! then has to transform keys
itself when a collection is passed to preserve the new behavior
introduced by #486.

[dhh]

Great work @tf! One question: Why did you update jbuilder for your app in the first place? By explicitly calling bundle update jbuilder or by accident?

[dhh]

Released as 2.11.2.

[tf]

jbuilder comes as a dependency of our pageflow engine gem using a (SemVer)-optimistic version constraint. Updating the engine in one of the host applications during development also triggered the minor version update. So things only ever broke on my local machine.