Update SocketIO.java

<b>public String getHeader had logic error  !</b> 
<i>
```
public String getHeader(String key) {
 		if (this.headers.contains(key)) // headers.contains(param) - get true if map 
                                        //has param value. this method do not  check KEYs in container  !
  			return this.headers.getProperty(key);
  		return null;
}
```
</i>

<b>I update it  like this : </b>
```
if (headers != null)
   return this.headers.getProperty(key);
```