Send JSON payloads properly

[jslag]

storyblok_ruby's call to RestClient::Resource was causing payloads for PUT and POST to be sent as application/x-www-form-urlencoded, rather than application/json. This was causing booleans to be interpreted as strings when updating stories.

This PR updates calls to match the to_json examples at https://github.com/rest-client/rest-client

Pull request type

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Other (please describe):

How to test this PR

• you'll need a story with a boolean value
• update the story via the API, something like

story_client = Storyblok::Client.new(oauth_token: 'TOKEN', api_url: 'mapi.storyblok.com', api_version: 1)
url = "spaces/SPACE/stories/STORY"
story = story_client.get(url)
payload = story['data']['story']
payload['name'] = 'something different'
story_client.put(url, { "story" => payload })

• reload the story, and check the Draft JSON. The previously boolean value will now be a string.

What is the new behavior?

With this PR, booleans stay boolean when stories are updated as above.