Fix Bowser constructor, add some tests

test/unit/bowser.js

@@ -0,0 +1,18 @@
+import test from 'ava';
+import Bowser from '../../src/bowser';
+import Parser from '../../src/parser';
+
+const UA = 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36 OPR/43.0.2442.1165';
+const browser = new Bowser(UA);
+
+test('Bowser`s constructor returns a Parser instance', (t) => {
+  t.truthy(browser instanceof Parser);
+});
+
+test('Bowser`s constructor fails if UA is empty', (t) => {
+  t.throws(() => (new Bowser()));
+});
+
+test('Bowser.parse parses UA and returns result', (t) => {
+  t.deepEqual(Bowser.parse(UA), browser.getResult());
+});