Problem with escaping HTML characters in Rails 3

Question

I use foo helper function in my view:

 <%= foo ["hello", "stack", "overflow"] %>

When foo is defined like this:

def foo(arr)
  result = ''
  arr.each do |a|
    result += content_tag(:div, a)
  end
  result
end

The page renders:

<div>hello</div><div>stack</div><div>overflow</div>

But, if change foo's definition to be:

def foo(arr)
  content_tag(:div, arr[0]) + content_tag(:div, arr[1]) + content_tag(:div, arr[2])
end

I get the expected result:

hello
stack
overflow

How would you fix foo's definition above to get the expected result ? (i.e. I don't want the characters to be escaped)