Scientist::Experiment.new creates Scientist::Default instance even when it should have been overridden

Hi friends! Thank you for your great work with this gem :sparkles:

I am having a problem in Rails apps where Scientist::Experiment defaults to the original `Default` object until the custom one is called - leading to some head-scratching about why the `try` block is not running even when `enabled?` is set to `true`.

I'm unsure whether this is a problem with the Rails load order because of how I've arranged my files, whether the examples in the README could be a little better, or whether there's genuinely a bug here.

```
$ bundle exec rails console
Loading development environment (Rails 5.1.6)
irb(main):001:0> Scientist::Experiment.new "something"
=> #
irb(main):002:0> LdapExperiment.new(name: "something")
=> #
irb(main):003:0> Scientist::Experiment.new "something"
=> #
```

I've followed the instructions in the README, which are delightful and comprehensive.

```ruby
# app/experiments/ldap_experiment.rb
require "scientist/experiment"

class LdapExperiment
include Scientist::Experiment

attr_accessor :name

def initialize(name:)
@name = name
end

def enabled?
# ...
end

def publish(result)
# ...
end
end

module Scientist::Experiment
def self.new(name)
LdapExperiment.new(name: name)
end
end
```

```ruby
# app/models/whatever.rb
class Whatever
include Scientist
def do_something
science "role lookup" do |e|
e.use { do_one_thing }
e.try { do_some_other_thing }
end
end
end
```

This is occurring in Rails 3.2.x and Rails 5.1.x applications, with version 1.2.0 of the gem.

使用 app/experiments/ldap_experiment.rb 和 issue 中所示的 Scientist::Experiment.new 调用，从 Rails 控制台复现该行为。阅读 scientist/experiment 和所引用的 README 示例，以确定这是加载顺序行为、实现 bug，还是文档歧义；当预期的覆盖行为得到确立，并由适当的修复或说明加以覆盖时，即视为完成。

由索引模型根据 Issue 内容生成。