Add Beacon DefaultValue and use non-nullable types

Following changes in #132 many or all nullable bool? may be changed to non-nullable bool when something like DefaultAttribute is used. E.g.

[Beacon("exf")] 
[DefaultValue(true)]
public bool IsFatal { get; set; }

[Beacon("ni")]
[DefaultValue(false)]
public bool NonInteractionHit { get; set; }

So, then the value == DefaultValue, then the property is not sent in the HTTP request.

Note: it may be applied to the TransactionRevenue, TransactionShipping, TransactionTax, too.

[maartenba]

Ok will close for now, but can always be reopened if we need it later.

[xmedeko]

I am ok with not changing. It’s a minor benefit for me.

I just disagree with:

• This way, with default value, the HTTP request is never bigger, but it may be smaller is the user sets the state, which is default.
• In essence, the protocol itself has three states, but the Google Analytics (server) has two states (1, 0). IMO the client states should be made by the server states, not by the protocol abilities. It’s not about nicer code only, but about the code clarity, too.