Issue summary

I'm doing:

ShopifyAPI::Context.setup(
  api_key:        SHOPIFY_API_KEY,
  api_secret_key: SHOPIFY_API_SECRET_KEY,
  host:           "ecsypno.com",
  scope:          "read_fulfillments, read_orders, read_customers",
  is_embedded:    false, # Set to true if you are building an embedded app
  api_version:    "unstable",
  is_private:     true, # Set to true if you have an existing private app,
  log_level: :debug
)

SESSION = ShopifyAPI::Auth::Session.new(
  shop:         "ecsypno.com",
  access_token: SHOPIFY_ACCESS_TOKEN,
)

client = CLIENT = ShopifyAPI::Clients::Rest::Admin.new(
  session: SESSION
)

# Clear all previous Webhooks, we're taking over.
client.get( path: 'webhooks' ).body['webhooks'].each do |hook|
  client.delete( path: "webhooks/#{hook['id']}" )
end

response = client.post(
  path: 'webhooks',
  body: {
    "webhook": {
      "topic":   "order/fulfillment_success",
      "address": OWN_URL,
      "format":  "json"
    }
  }
)

shopify_api version: unstable
Ruby version: ruby 2.7.7p221
Operating system: Linux xps 6.2.0-39-generic 40~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Thu Nov 16 10:53:04 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

         9: from /home/zapotek/workspace/scnr/license-manager/app.rb:46:in `<top (required)>'
         8: from .../gems/gems/sorbet-runtime-0.5.11163/lib/types/private/methods/_methods.rb:272:in `block in _on_method_added'
         7: from .../gems/gems/sorbet-runtime-0.5.11163/lib/types/private/methods/call_validation.rb:256:in `validate_call'
         6: from .../gems/gems/sorbet-runtime-0.5.11163/lib/types/private/methods/call_validation.rb:256:in `bind_call'
         5: from .../gems/gems/shopify_api-13.3.1/lib/shopify_api/clients/rest/admin.rb:84:in `post'
         4: from .../gems/gems/sorbet-runtime-0.5.11163/lib/types/private/methods/call_validation_2_7.rb:108:in `block in create_validator_method_fast1'
         3: from .../gems/gems/sorbet-runtime-0.5.11163/lib/types/private/methods/call_validation_2_7.rb:108:in `bind_call'
         2: from .../gems/gems/shopify_api-13.3.1/lib/shopify_api/clients/http_client.rb:56:in `request'
         1: from .../usr/lib/ruby/2.7.0/json/common.rb:156:in `parse'
.../usr/lib/ruby/2.7.0/json/common.rb:156:in `parse': 783: unexpected token at 'Bad Request' (JSON::ParserError)

Shopify / shopify-api-ruby

"Bad Request" instead of JSON when registering Webhook #1258

Issue summary

Expected behavior

Actual behavior

Steps to reproduce the problem