Correct and clean up `ViewDataDictionary` semantics

[dougbu]

Issues found during recent investigations:

• ViewDataDictionary.ModelMetadata.Model and ViewDataDictionary.Model can easily get out of sync
  • e.g. controller passes 23 to a view and ViewData.Model=24 executes in the view; later helpers (especially the lambda-based ones) will use ViewData.ModelMetadata.Model, which is still 23
• ViewDataDictionary.ModelMetadata==null isn’t always true when ViewDataDictionary.Model==null
  • in the false cases, ViewDataDictionary.ModelMetadata.ModelType may not match the containing page’s @model type (a corner case)
  • generally ViewDataDictionary contract isn't clear nor implemented consistently
• ViewDataDictionary<int>.Model will throw an NRE if it is read before it’s written
  • should return 0 in this case
• some calls to the ViewDataDictionary and ViewDataDictionary<TModel> constructors (e.g. from page activation) are incorrect for the intended semantics
  • “incorrect” is usually limited to ViewDataDictionary.ModelState being empty but could, in corner cases, lead to unwarranted errors about invalid model values
• the recent fix for Provide correct ViewDataDictionary<TModel>.ModelMetadata when Model is null #1426 (in Beta1) likely has some corner cases
  • should now provide a more robust fix for Provide correct ViewDataDictionary<TModel>.ModelMetadata when Model is null #1426

[dougbu]

/cc @danroth27 @yishaigalatzer I'm moving this into current sprint as discussed today. We have a fix for all the issues listed above. And it'll be good to get that in soon to see if the fix itself has any corner cases.

[dougbu]

Fixed w/ five commits especially the first, a3b07da through 5420b9d