fix: request_options middleware overriding spider options

[suzdalnitski]

Currently, any options set in the Crawly.Request will get overriden by the RequestOptions middleware. This PR fixes that to allow merging of both request options.

Crawly.Request.new(
  "http://example.com",
  headers,
  # the body option here will get overriden by the middleware from the config
  body: body
)

config :crawly,
  middlewares: [
    {Crawly.Middlewares.RequestOptions, proxy: {'https://proxy.com', 3128}}
  ],