Fix semantics of ExistenceRequirement::KeepAlive. (#3796)

* Fix semantics of ExistenceRequirement::KeepAlive.

* Bump runtime version

substrate/srml/support/src/traits.rs

@@ -156,6 +156,9 @@ impl<Imbalance: Drop> OnUnbalanced<Imbalance> for () {
 #[derive(Copy, Clone, Eq, PartialEq)]
 pub enum ExistenceRequirement {
 	/// Operation must not result in the account going out of existence.
+	///
+	/// Note this implies that if the account never existed in the first place, then the operation
+	/// may legitimately leave the account unchanged and still non-existent.
 	KeepAlive,
 	/// Operation may result in account going out of existence.
 	AllowDeath,