title: white space issue in isAlpha() function of express-validator
body: <p>I am using express-validator in my project
  <strong>my json from the client is</strong></p>
  
  <pre><code>{"name": "john doe"}
  </code></pre>
  
  <p><strong>my express validation code is</strong></p>
  
  <pre><code>[check('name', 'invalid name').isAlpha()]
  </code></pre>
  
  <p>why this code is returning <strong><em>invalid name</em></strong> while this is a string.
  Also I tried <strong>isString()</strong> but it is also not working it is working in the same style as <strong>isAlpha()</strong>.
   Error <strong>json response to the client is</strong> </p>
  
  <pre><code>{
  "errors": [
      {
          "value": "john doe",
          "msg": "invalid name",
          "param": "name",
          "location": "body"
      }
  ]
  }
  </code></pre>
  
  <p><strong>does isAlpha() function consider only one word as a string</strong></p>
  
  <p><strong>How can I fix this</strong></p>