Don't dig. (#128)

`dig` throws a `TypeError` exception when `params[scope]` is set (e.g. to a string).

Example

this works:
```ruby
params = { actor: { name: "Nicolas Cage" } }

# digging with keys that don't exist
params.dig(:foo, :bar)
=> nil

# digging with keys that partially exist
params.dig(:actor, :name, :whatever)
=> TypeError: String does not have #dig method
```

Full article:
http://anamaria.martinezgomez.name/2018/01/07/ruby-dig.html

Original discussion:
https://bugs.ruby-lang.org/issues/11762

lib/invisible_captcha/controller_ext.rb

@@ -89,7 +89,7 @@ def honeypot_spam?(options = {})
         # If honeypot is defined for this controller-action, search for:
         # - honeypot: params[:subtitle]
         # - honeypot with scope: params[:topic][:subtitle]
-        if params[honeypot].present? || params.dig(scope, honeypot).present?
+        if params[honeypot].present? || (params[scope] && params[scope][honeypot].present?)
           warn_spam("Honeypot param '#{honeypot}' was present.")
           return true
         else
@@ -99,7 +99,7 @@ def honeypot_spam?(options = {})
         end
       else
         InvisibleCaptcha.honeypots.each do |default_honeypot|
-          if params[default_honeypot].present? || params.dig(scope, default_honeypot).present?
+          if params[default_honeypot].present? || (params[scope] && params[scope][default_honeypot].present?)
             warn_spam("Honeypot param '#{scope}.#{default_honeypot}' was present.")
             return true
           end