spellCheck attr should only accept boolean

Closes #758

Author: japgolly

core/src/main/scala/japgolly/scalajs/react/vdom/HtmlAttrs.scala

@@ -1757,7 +1757,7 @@ trait HtmlAttrs {
     * This enumerated attribute defines whether the element may be checked for
     * spelling errors.
     */
-  final def spellCheck = VdomAttr("spellCheck")
+  final def spellCheck = VdomAttr[Boolean]("spellCheck")
 
   /**
     * If the value of the type attribute is image, this attribute specifies a URI

doc/changelog/1.7.5.md

@@ -10,3 +10,5 @@
     ^.onKeyDown ==> handler2,
   ```
 
+* The `spellCheck` attribute now only accepts booleans.
+