Removed unnecessary call to 'convert_key' in 'HashWithIndifferentAccess#to_hash'

All the keys are already Strings by virtue of being a `HashWithIndifferentAccess`.

I ran a simple benchmark and observed a 4% performance increase when calling the `to_hash` method.

    Benchmark.ips do |x|
      x.time = 30
      x.warmup = 5

      hash = {
        :a  => 'a',
        'b' => { :b1 => 'b1', 'b2' => { 'b3' => 'b3' } },
        :c  => 'c'
      }.with_indifferent_access

      x.report("to_hash") do
        hash.to_hash
      end
    end