Element: Fix setAttribute()

According to [^dom] all parameters are required. Currently, it is possible to skip the arguments without an error being issued during compilation. [^dom]: http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-F68F082

Author: tindzk

src/main/scala/org/scalajs/dom/lib.scala

@@ -751,7 +751,7 @@ class Element extends Node with NodeSelector with ParentNode {
  *
  * MSN
  */
- def setAttribute(name: String = ???, value: String = ???): Unit = ???
+ def setAttribute(name: String, value: String): Unit = ???
 
  /**
  * removeAttributeNS removes the specified attribute from an element.