Fix bug of calculation for redirect time in summary.

In PerformanceHelper#munge, redirect time was calculated according to the following expresion.

```ruby
hash[:summary][:redirect] = hash[:timing][:redirect_end] - hash[:timing][:redirect_end]
```

so the redirect time is always zero.

I fixed this to
```ruby
hash[:summary][:redirect] = hash[:timing][:redirect_end] - hash[:timing][:redirect_start]
```