fix(combine): fix CombineFactorySignature

Was lacking some important pieces.

For issue #28.

src/core.ts

@@ -96,18 +96,22 @@ export interface CombineProjectFunction {
 export interface CombineFactorySignature {
   <T1, T2, R>(
     project: (t1: T1, t2: T2) => R,
+    stream1: Stream<T1>,
     stream2: Stream<T2>): Stream<R>;
   <T1, T2, T3, R>(
     project: (t1: T1, t2: T2, t3: T3) => R,
+    stream1: Stream<T1>,
     stream2: Stream<T2>,
     stream3: Stream<T3>): Stream<R>;
   <T1, T2, T3, T4, R>(
     project: (t1: T1, t2: T2, t3: T3, t4: T4) => R,
+    stream1: Stream<T1>,
     stream2: Stream<T2>,
     stream3: Stream<T3>,
     stream4: Stream<T4>): Stream<R>;
   <T1, T2, T3, T4, T5, R>(
     project: (t1: T1, t2: T2, t3: T3, t4: T4, t5: T5) => R,
+    stream1: Stream<T1>,
     stream2: Stream<T2>,
     stream3: Stream<T3>,
     stream4: Stream<T4>,

tests/factory/combine.ts

@@ -19,6 +19,24 @@ describe('xs.combine', () => {
     });
   });
 
+  it('should have correct TypeScript signature', (done) => {
+    const stream1 = xs.create<string>({
+      start: listener => {},
+      stop: () => {}
+    });
+
+    const stream2 = xs.create<string>({
+      start: listener => {},
+      stop: () => {}
+    });
+
+    const combined = xs.combine(
+      (a, b) => a.slice(2) + b.slice(2),
+      stream1, stream2
+    );
+    done();
+  });
+
   it('should complete only when all member streams have completed', (done) => {
     const stream1 = xs.periodic(30).take(1);
     const stream2 = xs.periodic(50).take(4);