Review and update form parsing logic

[Tratcher]

src/Microsoft.AspNet.WebUtilities/ParsingHelpers.cs has at least one place where it calls char.IsWhiteSpace, but this allows many strange Unicode characters. Review this parsing code and update it to be more accurate.

[Tratcher]

Query string parsing also uses this code path and needs validation.

[davidfowl]

Moving this to beta6

[glennc]

Is this something that @blowdart should care about?

[blowdart]

Yea, I'd like to see the PR when it's done, although if you're narrowing to just space and tab I'm not so concerned.

It is worth reviewing where other things call IsWhiteSpace when parsing user input.

[muratg]

Moving out of V1. We're open to PRs @blowdart :)

[blowdart]

Yea, we all know you don't want me issuing PRs for anything other than Docs 👍

[aspnet-hello]

This issue was moved to dotnet/aspnetcore#2733