fix(radio): radio w/ falsy value not checkable

Closes #5348

ionic/components/radio/radio-group.ts

@@ -103,7 +103,7 @@ export class RadioGroup {
       let oldVal = this.value;
 
       // set the radiogroup's value
-      this.value = val || '';
+      this.value = isDefined(val) ? val : '';
 
       this.updateValue();
 

ionic/components/radio/test/basic/index.ts

@@ -34,6 +34,13 @@ class E2EApp {
 
     this.relationship = 'enemies';
 
+
+    this.items = [
+      { description: 'value undefined', value: undefined },
+      { description: 'value false string', value: 'false' },
+      { description: 'value false boolean', value: false },
+      { description: 'value 0', value: 0 },
+    ];
   }
 
   setApple() {

ionic/components/radio/test/basic/main.html

@@ -99,4 +99,18 @@
     <code><b>pet:</b> {{pet}}</code>
   </div>
 
+  <ion-list radio-group [(ngModel)]="someNumber">
+    <ion-item *ngFor="#item of items">
+      <ion-label>
+        {{ item.description }}
+      </ion-label>
+​
+      <ion-radio [value]="item.value"></ion-radio>
+    </ion-item>
+  </ion-list>
+
+  <div padding>
+    <code><b>someNumber:</b> {{someNumber}}</code>
+  </div>
+
 </ion-content>