ValueError at /blog/password_change/blue/
   The view blog.views.password_change didn't return an HttpResponse object. It returned None instead.

url(r'^password_change/(?P<username>[-w.]+)/$', views.password_change, name='password_change'),
url(r'^password_change_done/$', views.password_change_done, name='password_change_done'),

class PasswordChangeForm(SetPasswordForm):
error_messages = dict(SetPasswordForm.error_messages, **{
    'password_incorrect': ("Your old password was entered incorrectly. Please enter it again."),
})
oldpassword = forms.CharField(
    label=("Old password"),
    strip=False,
    widget=forms.PasswordInput(attrs={'autofocus': True}),
)

field_order = ['oldpassword', 'password1', 'password2']

def clean_oldpassword(self):
    oldpassword = self.cleaned_data["oldpassword"]
    if not self.user.check_password(oldpassword):
        raise forms.ValidationError(
            self.error_messages['password_incorrect'],
            code='password_incorrect',
        )
    return oldpassword

@login_required
def password_change(request, username):
    if request.method == 'POST':
        form = PasswordChangeForm(data=request.POST, user=request.user)
        if form.is_valid():
           oldpassword = form.cleaned_data.get('oldpassword')
           password1 = form.cleaned_data.get('password1')
           password2 = form.cleaned_data.get('password2')
           if password1 == password2:
               update_session_auth_hash(request, form.username)
               form.save()
               return HttpResponseRedirect('/blog/password_change_done/')
           else:
               return render(request, 'blog/detail.html', {'error_message': 'password mismatch'})

    else:
        form = PasswordChangeForm(user=request.user)
        return redirect(reverse('blog:profile', args=[form.user.get_username()]))