randym/activeadmin-axlsx — pull request #24 by morr (2014-01-22 10:02:03+00:00)
Title: 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 is accessible here
end
end

index do
column :date do |v|
format_date v.date # and of course 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
```