Fix bug in non-boolean attributes being collapsed

665a075 introduced a change where non-boolean HTML attributes
(such as `title="title"`) would be stringified as booleans (such as
`title`) if their value was the same as their attribute name.

lib/element.js

@@ -139,25 +139,33 @@ function attribute(ctx, key, value) {
   var schema = ctx.schema
   var space = schema.space
   var info = find(schema, key)
-  var name
+  var name = info.attribute
+
+  if (info.overloadedBoolean && (value === name || value === '')) {
+    value = true
+  } else if (
+    info.boolean ||
+    (info.overloadedBoolean && typeof value !== 'string')
+  ) {
+    value = Boolean(value)
+  }
 
   if (
     value == null ||
     value === false ||
-    (typeof value === 'number' && isNaN(value)) ||
-    (!value && info.boolean)
+    (typeof value === 'number' && isNaN(value))
   ) {
     return EMPTY
   }
 
-  name = attributeName(ctx, info.attribute)
+  name = attributeName(ctx, name)
 
-  if (value === true || (value && info.boolean)) {
-    value = name
-  }
+  if (value === true) {
+    if (space === 'html') {
+      return name
+    }
 
-  if (space === 'html' && value === name) {
-    return name
+    value = name
   }
 
   return name + attributeValue(ctx, key, value, info)