fixes #58 Tuple2 equality is broken

tuple2 equality was broken if the first field of the second tuple was
falsy. Bad falsy if statement :(

src/Tuple2.ts

@@ -105,7 +105,7 @@ export class Tuple2<T,U> implements Value {
         if (<any>other === this) {
             return true;
         }
-        if (!other || !other._fst) {
+        if (!other || other._fst === undefined) {
             return false;
         }
         contractTrueEquality("Tuple2.equals", this, other);

tests/Tuple2.ts

@@ -29,4 +29,10 @@ describe("Tuple2 manipulation", () => {
         Option.of(Tuple2.of(1,2)).equals(Tuple2.ofArray<number,number>([1,2]))));
     it("build from array works - failure", () => assert.ok(
         Tuple2.ofArray([1,2,3]).isNone()));
+    it("gives correct equality answer even if fst is falsy", () => assert.ok(
+        Tuple2.of(0, "").equals(Tuple2.of(0, ""))));
+    it("gives correct equality answer #2", () => assert.ok(
+        !Tuple2.of(0, "").equals(Tuple2.of(0, "a"))));
+    it("gives correct equality answer #3", () => assert.ok(
+        !Tuple2.of(1, "").equals(Tuple2.of(0, ""))));
 });