Bundle OpenStream shoud return ConcatenatedAssets if has more than one asset, even in debug mode

In debug mode, with the default configuration assets won't get
concatenated, which is a good thing. But in some cases (e.g.: using
Bundles.Url for downlading a script async), even in debug mode, if you
ask for a bundle url, it has to be concatenated. It has no sense to
serve the first asset of the bundle.

(Note: I don't know why there is a big diff, only changed:
1. Bundle.cs:
      *method OpenStream modified (line 137) 
      *Added property ConcatenateAssetsDefaultSeparator (line 26)
2. ScriptBundle.cs: override property ConcatenateAssetsDefaultSeparator  (line 52)
3. WebHostTests.cs: added the Test GivenDebugMode_BundleAUrlShouldServeConcatenatedScripts (117)