Fix for update by reference of default variable values

[aurelijusbanelis]

So Field "..." argument "..." of type "...!" is required but not provided. validation would always work, despite the order of the calls.

Context of the issue

# Note: there is default enum value in variables
query SomeOperation ($locale: Locale! = DE) {
	myAction(myEnum: $locale) {
		id
	}
}

query SomeOperation {
	# Note: Not providing mandatory parameter: (myEnum: Locale!)
	myAction {
		id
	}
}

• Before fix: Missing field not caught
• After fix: Field "myAction" argument "myEnum" of type "Locale!" is required but not provided.

Cause

VariablesInAllowedPosition rule updates value.ExpectedType.NonNull = false, resulting in ProvidedRequiredArguments rule to have argDef.Type.NonNull changed on the next call.

[coveralls]

Coverage increased (+0.2%) to 92.342% when pulling 76372a2 on aurelijusbanelis:by-reference into 2a3d320 on vektah:master.

[aurelijusbanelis]

May be duplicate of #158

[aurelijusbanelis]

@lwc I saw you have merged #158

So I left only test cases to catch similar mistakes in the future.

[StevenACoffman]

Thanks!