Make it so that the activemodel validations return the correct error types

[alexcarruthers]

We were using this gem and getting errors returned that looked like:
#<ActiveModel::Error attribute=weight, type=-25 kg must be >= 0 kg, options={}>
where the type was not deterministic. Looking into the gem and it looks like the message is being sent as a type. The documentation for ActiveRecord::Errors.add states that if a string is passed into type, it's used as the error message, but in this case it doesn't seem to do that. This PR updates the code to be more explicit, while also passing on the actual error type that's violated.

[kmcphillips]

Thanks for the good test coverage.

[paracycle]

Ummm, let's get this merged, since it is obviously better and more explicit code. But now that I am looking at the Rails code, this might be pointing to a regression that went undetected for many versions now?!

[paracycle]

... The documentation for ActiveRecord::Errors.add states that if a string is passed into type, it's used as the error message, but in this case it doesn't seem to do that.

Small correction: this is not broken, this is still happening, since the message attribute of an ActiveModel::Error is computed lazily to read either from the type value or the string message passed.

The thing that is unexpected is for the type field to also have the message as its value.

Example:

irb> require "active_model"
irb>
irb> class Foo
irb>   include ActiveModel::Model
irb>
irb>   attr_reader :name
irb>
irb>   validates_each :name do |r, a, v|
irb>     r.errors.add(:name, "this is not a message")
irb>   end
irb> end
#=> [ActiveModel::BlockValidator]

irb> foo = Foo.new
#=> #<Foo:0x000000012a83cce8>

irb> foo.valid?
#=> false

irb> foo.errors
#=> #<ActiveModel::Errors [#<ActiveModel::Error attribute=name, type=this is not a message, options={}>]>

# The following works as expected
irb> foo.errors.first.message
#=> "this is not a message"

# The following is surprising
irb> foo.errors.first.type
#=> "this is not a message"