Avoid NullPointerException in isEnabled method

Added null validation of readonly variable in protected boolean  isEnabled(String locator) method. Please see issue https://github.com/MarkusBernhardt/robotframework-selenium2library-java/issues/53

Basically, this avoids a NullPointerException if the html element doesn't have the "readonly" attribute.

Use this Robot test to check it:
```
*** Settings ***
Library  Selenium2Library

*** Test Cases ***

Test Should Be Pass and Should Not Throw a RuntimeException
    Open Browser    http://www.zkoss.org/zkdemo/input/form_sample    firefox    mainbrowser
    Wait Until Keyword Succeeds  10 sec  1 sec  Element Should Be Enabled  //input[../../../td[1]//span[contains(text(), 'Password')]]
    Close All Browsers
```

I am available for any questions you have.

Thank you
Hi,

have been out of office for some time. Will look into all the thing you made tomorrow.

Cheers,
Markus

Am 22.06.2014 um 20:31 schrieb atcarmo <notifications@github.com>:

> In src/main/java/com/github/markusbernhardt/selenium2library/keywords/Element.java:
> 
> > @@ -1572,7 +1572,7 @@ protected boolean isEnabled(String locator) {
> >  			return false;
> >  		}
> >  		String readonly = element.getAttribute("readonly");
> > -		if (readonly.equals("readonly") || readonly.equals("true")) {
> > +		if (readonly != null && (readonly.equals("readonly") || readonly.equals("true"))) {
> Line 1574 might return null, that's why we need this validation.
> 
> —
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