Feature: rendering optional attributes

[kortirso]

@jho406 in continue #30

json.optional! - defines attribute or structure and marks it as optional, during serialization this method checks condition and renders or does not render such attribute

example of usage

json.optional! :properties do
  json.optional! :value, -> { object.value } # lambda here is to avoid value calculations before checking conditions for rendering
end

Works with partials, I just think how optionals can be used in simple template

[jho406]

Thanks for this PR! Out of the 3 PRs, i think this is one that i'm not fully ready for. Even if its manual, I do prefer the explicitness of if statements so I would actually do

if object.value
  json.properties do
    json.value object.value
  end
end

Other thoughts I have are.

1. Does it work with digging?
2. If i can mix json.optional! with the normal setters like json.header. For example, I wonder if this would work

json.optional! :properties do
  json.greeting "hello"
end

I think i'll leave this PR open for a bit to see if there are other commenters, but I think your approach in the meantime is a good path, extend props_template.

I also wonder if we can modify the syntax somehow using the block extension manager:

json.properties(optional: true) do
    json.value(object.value) if object.value
end