test: cast string | number to string before concatenating

T of this Observable is string | number, so TypeScript does not allow
using +. The desired behaviour for this function is to concatenate the
string values, so cast to strings. On master, these were incorrectly
inferred as never, which is why this error was not surfaced.

spec/operators/zipAll-spec.ts

@@ -39,7 +39,7 @@ describe('zipAll operator', () => {
       of('a', 'b', 'c'),
       of(1, 2, 3)
     )
-    .pipe(zipAll((a, b) => a + b))
+    .pipe(zipAll((a, b) => String(a) + String(b)))
     .subscribe((x) => {
       expect(x).to.equal(expected[i++]);
     }, null, done);
@@ -378,7 +378,7 @@ describe('zipAll operator', () => {
       of('a', 'b', 'c'),
       of(1, 2)
     )
-    .pipe(zipAll((a, b) => a + b))
+    .pipe(zipAll((a, b) => String(a) + String(b)))
     .subscribe((x) => {
       expect(x).to.equal(expected[i++]);
     }, null, done);