4.x: simplify default(X) usages

[akarnokd]

• Simplify default(X) where X is inferable from the usage.
• Aggregate:
  • don't initialize to default values
  • clear accumulation upon failure or completion
• LastAsync:
  • don't initialize to default values
  • clear accumulation upon failure or completion
• LastOrDefaultAsync:
  • don't initialize to default values
  • clear accumulation upon failure or completion
• Upgrade Test.System.Reactive project to C# 7.1

[clairernovotny]

This should be removed. The LangVersion is set to latest in the Directory.Build.props. This lowers it.