$interpolate: coverage + cast to string on single token

Author: yyx990803

src/api/data.js

@@ -119,7 +119,7 @@ exports.$interpolate = function (text) {
   var vm = this
   if (tokens) {
     if (tokens.length === 1) {
-      return vm.$eval(tokens[0].value)
+      return vm.$eval(tokens[0].value) + ''
     } else {
       return tokens.map(function (token) {
         return token.tag

test/unit/specs/api/data_spec.js

@@ -145,6 +145,7 @@ describe('Data API', function () {
 
   it('$interpolate', function () {
     expect(vm.$interpolate('abc')).toBe('abc')
+    expect(vm.$interpolate('{{a}}')).toBe('1')
     expect(vm.$interpolate('{{a}} and {{a + b.c | double}}')).toBe('1 and 6')
   })