Updates the form class names to use rails pattern

[CaiqueMitsuoka]

The problem

When creating the form field with the Attachy helper, things worked fine for classes with single word names, like User, with the #downcase method. But when the class had a composed name like AdminUser the result would be adminuser since #downcase only replaces uppercase chars with the downcased version.

Before example:

## View code
## ...
<%= f.attachy :avatar %>
<%= f.text_field :name %>

## Controller params inspect
params
# Output: <ActionController::Parameters {
#   "adminuser" => {
#     "avatar" => [...]
#   },
#   "admin_user"=> {
#     "name" => "Foo Bar"
#    }
# } permitted: false>

The solution

Rails uses the #underscore method to generate the downcased underscore separed class names to generate form. So when composed words classes use the attachy helper the field would come in a different form data.

After example

## View code
## ...
<%= f.attachy :avatar %>
<%= f.text_field :name %>

## Controller params inspect
params
# Output: <ActionController::Parameters {
#   "admin_user"=> {
#     "name" => "Foo Bar"
#     "avatar" => [...]
#    }
# } permitted: false>