Allow raw/native JSON in JS config object, e.g. checkIgnore function

[waltjones]

Fixes #648
Fixes #710

Use Rollbar::JSON::Value to pass native JSON values, without enclosing string quotes.

Example:

Rollbar.configure do |config|
  config.js_enabled = true
  config.js_options = {
    accessToken: "POST_CLIENT_ITEM_ACCESS_TOKEN",
    checkIgnore: Rollbar::JSON::Value.new('function(){ alert("bar") }')
  }
end

Ruby JSON.generate vs. ActiveSupport #to_json and #as_json

When serializing the js_options object, the middleware now uses Ruby JSON.generate instead of ActiveSupport's #to_json.

Early versions of ActiveSupport could use the #encode_json method to get a similar ability to inject values without enclosing quote marks.

ActiveSupport before version 4.1.0 supported the #encode_json method, and then the activesupport-json_encoder gem did for Rails < 5.x. For Rails 5.x, the desired functionality doesn't appear achievable via ActiveSupport. For all versions, the dependencies and requirements are fragmented, and the functionality would not work at all on a Rack app without ActiveSupport.

The native Ruby JSON#generate works consistently across all versions of Ruby and imposes no dependencies on users of the gem. The encoder requirement is only on the generation of the js_options object in the middleware and has no impact on other JSON objects in the gem, or on the host application.

[ArturMoczulski]

LGTM :) Feel free to merge