title: RequiredFieldValidator with ValidationSummary not working with enter key inside empty textbox
body: <p>Consider the following ASPX:</p>
  
  <pre><code>&lt;asp:ValidationSummary ID="vSummary" runat="server" CssClass="error-message" HeaderText="&lt;p&gt;Please correct the following errors with your profile.&lt;/p&gt;" ShowMessageBox="false" DisplayMode="BulletList" ShowSummary="true" /&gt;
  
  &lt;asp:TextBox ID="txtTest" runat="server" ClientIDMode="Static"&gt;&lt;/asp:TextBox&gt;
  &lt;asp:RequiredFieldValidator ID="rfv" runat="server" Display="None" ErrorMessage="Enter a value in the textbox." ControlToValidate="txtTest"&gt;&lt;/asp:RequiredFieldValidator&gt;
  
  &lt;asp:Button ID="btn1" runat="server" Text="Button 1" OnClick="btn1Click" ClientIDMode="Static" /&gt;
  </code></pre>
  
  <p>With text entered into the textbox, everything behaves as expected.</p>
  
  <p>Clicking the button submits the form / pressing 'Enter' while inside the textbox submits the form.</p>
  
  <p>However, when the textbox is empty, pressing 'enter' does nothing.  Clicking the button will display the validation summary, as expected, but pressing 'enter' does not.  </p>
  
  <p>How can I make the validation summary display after the 'Enter' key is pressed inside an empty textbox?</p>