title: Selenium Desired Capabilities - set handlesAlerts for PhantomJS driver
body: <p>I'm trying out phantomJS with webdriver and I'm having trouble with handling javascript alerts.  I notice the phantomjs driver desired_capabilities has a field <code>'handlesAlerts': False</code>  Is there a way to set this value to true?  I've tried the obvious way but that doesn't have any effect:</p>
  
  <pre><code>drv = webdriver.PhantomJS(desired_capabilities={'handlesAlerts': True})
  
  print drv.desired_capabilities
  
  {u'browserName': u'phantomjs',
   u'driverName': u'ghostdriver',
   u'driverVersion': u'1.0.3',
   u'handlesAlerts': False,
   u'javascriptEnabled': True,...}
  </code></pre>
  
  <p>I can change the value in the dictionary <code>drv.desired_capabilities['handlesAlerts'] = True</code>, but when I try to switch to an alert, I get an error message.</p>
  
  <pre><code>$cat index.html 
  &lt;html&gt;
  &lt;body&gt;
  &lt;script type="text/javascript"&gt;
      alert('FOO!');
  &lt;/script&gt;
      Hello World.
  &lt;/body&gt;
  &lt;/html&gt;
  
  &gt;&gt;&gt; from selenium import webdriver
  &gt;&gt;&gt; driver = webdriver.PhantomJS()
  &gt;&gt;&gt; driver.desired_capabilities['handlesAlerts'] = True
  &gt;&gt;&gt; driver.get('index.html')
  &gt;&gt;&gt; alert = driver.switch_to_alert()
  &gt;&gt;&gt; alert.text
  
  Traceback (most recent call last):
  &lt;snip&gt;
  selenium.common.exceptions.WebDriverException: Message: 
     'Invalid Command Method -  Request    =&gt; 
                   {"headers":{"Accept":"application/json",
                                "Accept- Encoding":"identity",
                                "Connection":"close",
                                "Content-Type":"application/json;charset=UTF- 8",
                                "Host":"127.0.0.1:56009", 
                                "User-Agent":"Python- urllib/2.7"},
                    "httpVersion":"1.1",
                    "method":"GET",
                    "url":"/alert_text",
                    "urlParsed": {"anchor":"",
                                  "query":"",
                                  "file":"alert_text",
                                  "directory":"/",
                                  "path":"/alert_text",
                                  "relative":"/ alert_text",
                                  "port":"",
                                  "host":"",
                                  "password":"",
                                  "user":"",
                                  "userInfo":"",
                                  "authority":"",
                                  "protocol ":"",
                                  "source":"/alert_text",
                                  "queryKey":{},
                                  "chunks":["alert_text"]},
                                  "urlOriginal":"/session/cd31ed90-a5f8-11e2-856d-5783db9f5342/alert_text"}' 
  </code></pre>