Default value lambda improvements

[natesalisbury]

Improvements to using a lambda as a default value:

• Don't type_cast the actual lambda.
• Type cast the result of calling a default lambda.

[coveralls]

Changes Unknown when pulling 08bd9ff on natesalisbury:default_value_lambdas into ** on aws:master**.

[coveralls]

Changes Unknown when pulling 08bd9ff on natesalisbury:default_value_lambdas into ** on aws:master**.

[coveralls]

Changes Unknown when pulling 08bd9ff on natesalisbury:default_value_lambdas into ** on aws:master**.

[coveralls]

Changes Unknown when pulling b5e0e8f on natesalisbury:default_value_lambdas into ** on aws:master**.

[natesalisbury]

@awood45 any feedback for these changes?

[awood45]

Sorry, haven't had a chance to review yet. It's on my work backlog.

[natesalisbury]

No worries, thanks!

[metaskills]

Was interested in using default values today and was surprised that this was not the behavior. In fact, would this PR solve a bug? I am not sure why storing a proc as a string would be the expected results?

[metaskills]

Like, is this the expected way to use a default value for a hash key?

class MyModel
  include Aws::Record
  string_attr       :prefix, hash_key: true
  string_attr       :description

  def initialize(*args)
    super
    self.prefix ||= new_prefix
  end

  # ...
end

[alextwoods]

Apologies it has taken us so long to get around to reviewing this change. Overall it looks good, however, in ItemData#populate_default_values, the default_value lambda will end up getting called twice.

I'm happy to fix that later today.