Fix for Issue 316. Crash: 'Fatal Error: Can't form Range with end < start

[danhollywells]

requestedCount incremented beyond the prefetchedResorces count. See Issue #316

Fix: eliminate requestedCount

Because the start() method captures the initial X Resources by index and ships them off to startPrefetchingResource() and that methods recursive behavior, you can end up in a state where all the items have been processed but the initial X resources will get double processed because they were captured in the for loop in the start() method and again in the recursion of the startPrefetchingResource(). So if during the prefetch you get into this state and you call stop() you will end up with an unbalanced Range when sorting out the failedResources.

[codecov-io]

Current coverage is 74.27% (diff: 96.96%)

Merging #422 into master will decrease coverage by 0.22%

@@             master       #422   diff @@
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  Files            14         14          
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- Hits           1262       1256     -6   
- Misses          432        435     +3   
  Partials          0          0          

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[onevcat]

LGTM! Thanks for pointing this out!