Ruby's Hash#merge with an optional block to resolve key conflicts.

hash_merge_block.rb

# via @avdi's RubyTapas Episode 170: http://www.rubytapas.com/episodes/170-Hash-Merge

headers = <<END
Accept: */*
Set-Cookie: foo=42
Set-Cookie: bar=23
END

def parse_headers(headers)
  headers.lines.reduce({}) { |result, line|
    name, value = line.split(':')
    result.merge(name.strip => value.strip)
  }
end

parse_headers(headers)
#=> {"Accept"=>"*/*", "Set-Cookie"=>"bar=23"}

def parse_headers(headers)
  headers.lines.reduce({}) { |result, line|
    name, value = line.split(':')
    result.merge(name.strip => value.strip) { |key, left, right| 
      Array(left) + Array(right)
    }
  }
end

parse_headers(headers)
#=> {"Accept"=>"*/*", "Set-Cookie"=>["bar=42", "bar=23"]}