title: updating "placeholder" attribute using knockout
body: <p>I have a form with some fields getting some data using knockout.js (ver. 2.1.0).
  For example, to update the "value" field of an input I put:</p>
  
  <pre><code>&lt;input type="text"  name="contrasena" id="login-user" value="" placeholder="" data-bind="value: user"&gt;
  </code></pre>
  
  <p>I have a JSON to store the value a I want to use for "pass" keyword, and it works correctly.</p>
  
  <p>I tried to set "placeholder" attribute using the same method, but it doesn't works:</p>
  
  <pre><code>&lt;input type="text"  name="contrasena" id="login-user" placeholder="" data-bind="placeholder: user"&gt;
  </code></pre>
  
  <p>I tried to modify knockout.js file adding "ko.bindingHandlers['placeholder']" function based on "ko.bindingHandlers['value']" (modifying "placeholder" instead of "value" in "ko.jsonExpressionRewriting.writeValueToProperty" function), but it doesn't work correctly, it puts the information in "value" attribute instead of "placeholder".</p>
  
  <p>Anyone knows the way to solve this?</p>
  
  <p>Thank you very much!</p>