Prior to this commit, `ActionText::Content#inspect` called `#to_s` and
truncated the result to 25 characters. However, `#to_s` calls
`#to_rendered_html_with_layout` which, by default, renders the same
first 25 characters for every instance.
For example, with models such as:
```ruby
class Message < ActiveRecord::Base
has_rich_text :content
end
Message.create(content: "first message")
Message.create(content: "second message")
Message.create(content: "third message")
```
The output of `#inspect` is indistinguishable:
```irb
irb> Message.all.map { |message| message.content.body }
Rendered .../actiontext/app/views/action_text/contents/_content.html.erb within layouts/action_text/contents/_content (Duration: 2.4ms | Allocations: 881)
Rendered .../actiontext/app/views/action_text/contents/_content.html.erb within layouts/action_text/contents/_content (Duration: 0.7ms | Allocations: 257)
Rendered .../actiontext/app/views/action_text/contents/_content.html.erb within layouts/action_text/contents/_content (Duration: 0.6ms | Allocations: 257)
=> [#<ActionText::Content "<div class=\"trix-conte...">,
#<ActionText::Content "<div class=\"trix-conte...">,
#<ActionText::Content "<div class=\"trix-conte...">]
```
This commit changes `#inspect` to call `#to_html`, which does not render
the Action Text layout. So the output of `#inspect` will be:
```irb
irb> Message.all.map { |message| message.content.body }
=> [#<ActionText::Content "first message">,
#<ActionText::Content "second message">,
#<ActionText::Content "third message">]
```
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