Retain undefined in spreads w/strictNullChecks

Previously, both optional and `| undefined` caused spread properties to
combine with preceding properties and drop the `undefined`. Now, with
strictNullChecks, optional and `| undefined` properties still combine
with preceding properties but don't drop the `undefined`.

Author: sandersn

src/compiler/checker.ts

@@ -7640,11 +7640,11 @@ namespace ts {
                 if (members.has(leftProp.name)) {
                     const rightProp = members.get(leftProp.name);
                     const rightType = getTypeOfSymbol(rightProp);
-                    if (maybeTypeOfKind(rightType, TypeFlags.Undefined) || rightProp.flags & SymbolFlags.Optional) {
+                    if (rightProp.flags & SymbolFlags.Optional) {
                         const declarations: Declaration[] = concatenate(leftProp.declarations, rightProp.declarations);
                         const flags = SymbolFlags.Property | (leftProp.flags & SymbolFlags.Optional);
                         const result = createSymbol(flags, leftProp.name);
-                        result.type = getUnionType([getTypeOfSymbol(leftProp), getTypeWithFacts(rightType, TypeFacts.NEUndefined)]);
+                        result.type = getUnionType([getTypeOfSymbol(leftProp), rightType]);
                         result.leftSpread = leftProp;
                         result.rightSpread = rightProp;
                         result.declarations = declarations;