fix(radio): initialiase disabled state according to parent's state

Fixes #2635
Closes #2639

src/buttons/radio.spec.ts

@@ -1,10 +1,9 @@
-import {TestBed, ComponentFixture, async} from '@angular/core/testing';
-import {By} from '@angular/platform-browser';
-import {createGenericTestComponent} from '../test/common';
-
 import {Component} from '@angular/core';
-import {Validators, FormControl, FormGroup, FormsModule, ReactiveFormsModule, NgModel} from '@angular/forms';
+import {async, ComponentFixture, TestBed} from '@angular/core/testing';
+import {FormControl, FormGroup, FormsModule, NgModel, ReactiveFormsModule, Validators} from '@angular/forms';
+import {By} from '@angular/platform-browser';
 
+import {createGenericTestComponent} from '../test/common';
 import {NgbButtonsModule} from './buttons.module';
 
 const createTestComponent = (html: string) =>
@@ -324,6 +323,28 @@ describe('ngbRadioGroup', () => {
     expect(getInput(fixture.nativeElement, 0).hasAttribute('disabled')).toBeFalsy();
   });
 
+  it('should disable label and input when added dynamically in reactive forms', () => {
+    const forHtml = `
+      <form [formGroup]="disabledForm">
+        <div ngbRadioGroup formControlName="control">
+          <label ngbButtonLabel *ngIf="shown">
+            <input ngbButton type="radio" name="radio" [value]="'one'"/> One
+          </label>
+        </div>
+      </form>
+    `;
+
+    const fixture = createTestComponent(forHtml);
+    fixture.componentInstance.shown = false;
+    fixture.componentInstance.disabledForm.disable();
+    fixture.detectChanges();
+
+    fixture.componentInstance.shown = true;
+    fixture.detectChanges();
+    expect(getLabel(fixture.nativeElement, 0)).toHaveCssClass('disabled');
+    expect(getInput(fixture.nativeElement, 0).hasAttribute('disabled')).toBeTruthy();
+  });
+
   it('should disable label and input when it is disabled using template-driven forms', async(() => {
        const html = `
       <form>

src/buttons/radio.ts

@@ -1,4 +1,4 @@
-import {Directive, forwardRef, Input, Renderer2, ElementRef, OnDestroy} from '@angular/core';
+import {Directive, ElementRef, forwardRef, Input, OnDestroy, Renderer2} from '@angular/core';
 import {ControlValueAccessor, NG_VALUE_ACCESSOR} from '@angular/forms';
 
 import {NgbButtonLabel} from './label';
@@ -129,6 +129,7 @@ export class NgbRadio implements OnDestroy {
       private _group: NgbRadioGroup, private _label: NgbButtonLabel, private _renderer: Renderer2,
       private _element: ElementRef<HTMLInputElement>) {
     this._group.register(this);
+    this.updateDisabled();
   }
 
   ngOnDestroy() { this._group.unregister(this); }