Added new tests & missing return type to tpl

- Ensuring that various strings from en.json will work
- Added missing return type

ghost/tpl/lib/tpl.js

@@ -11,7 +11,7 @@ const interpolate = /(?<!{){([^{]+?)}/g;
  *
  * @param {String} string - string with optional {data properties}
  * @param {Object} [data] - optional data to interpolate
- * @returns
+ * @returns {string} the interpolated string
  */
 module.exports = (string, data) => {
     if (!data) {

ghost/tpl/test/tpl.test.js

@@ -20,7 +20,7 @@ describe('tpl', function () {
         result.should.eql('Go visit https://example.com');
     });
 
-    it('Can handle mixing with handlebars-related messages', function () {
+    it('Can mix interpolation handlebars in the same message', function () {
         const string = '{{#get}} helper took {totalMs}ms to complete';
         const data = {
             totalMs: '500'
@@ -30,6 +30,18 @@ describe('tpl', function () {
         result.should.eql('{{#get}} helper took 500ms to complete');
     });
 
+    it('Can mix interpolation with handlebars-block helpers without escaping', function () {
+        const string = '{{#{helperName}}} helper took {totalMs}ms to complete';
+
+        const data = {
+            helperName: 'get',
+            totalMs: '500'
+        };
+
+        let result = tpl(string, data);
+        result.should.eql('{{#get}} helper took 500ms to complete');
+    });
+
     it('Can handle escaped left braces', function () {
         const string = 'The \\{\\{{helperName}}} helper is not available.';
         const data = {