fix default gather input is digits and gather dtmf should not require speech

lib/tasks/gather.js

@@ -30,6 +30,10 @@ class TaskGather extends SttTask {
       'speechTimeout', 'timeout', 'say', 'play'
     ].forEach((k) => this[k] = this.data[k]);
 
+    // gather default input is digits
+    if (!this.input) {
+      this.input = ['digits'];
+    }
     /* when collecting dtmf, bargein on dtmf is true unless explicitly set to false */
     if (this.dtmfBargein !== false  && this.input.includes('digits')) this.dtmfBargein = true;