Ability to execute controller's view_context methods in column builder block

Currently `Axml::Builder` behaviour is not exactly the same as `CSVBuilder` logic.
`Axml::Builder`  does not have access to controller's `view_context` in column proc, but `CSVBuilder` does.
Let's fix it.

Code example:
```ruby
xlsx do
  clear_columns
  column :date do |v|
    # HERE! format_date (helper method defined in controller) is not accessible
    format_date v.date
  end
end

csv do
  column :date do |v|
    format_date v.date # but it works here
  end
end

index do
  column :date do |v|
    format_date v.date # and here
  end
end

controller do
  helper_method :format_date

  def format_date date
    # some helper method in controller
    case params[:grouping]
      when 'by_day' then date.strftime '%d.%m.%Y'
      when 'by_month' then date.strftime '%B %Y'
    end
  end
end
```