Era change can be forced on and forced off (#3374)

* Forcing can be on or off.

* Add a testcase.

substrate/node/runtime/src/lib.rs

@@ -80,8 +80,8 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to equal spec_version. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 139,
-	impl_version: 139,
+	spec_version: 140,
+	impl_version: 140,
 	apis: RUNTIME_API_VERSIONS,
 };