Fix value conversion method

In Solidus, this method has changed from having two parameters to
having three. This updates the method here to match, so we avoid
any too_many_parameter errors. Most noticably, this was affecting
our test suite.

See: solidusio/solidus@e05da68

app/models/solidus_paypal_braintree/gateway.rb

@@ -292,7 +292,7 @@ def to_hash(preference_string)
       JSON.parse(preference_string.gsub("=>", ":"))
     end
 
-    def convert_preference_value(value, type)
+    def convert_preference_value(value, type, preference_encryptor = nil)
       if type == :hash && value.is_a?(String)
         value = to_hash(value)
       end