fix: map class -> className

Fixes #606

.changeset/chatty-waves-invite.md

@@ -0,0 +1,5 @@
+---
+'markdown-to-jsx': patch
+---
+
+Handle `class` attribute from arbitrary HTML properly to avoid React warnings.

index.compiler.spec.tsx

@@ -2460,10 +2460,18 @@ describe('GFM tables', () => {
 
 describe('arbitrary HTML', () => {
   it('preserves the HTML given', () => {
-    render(compiler('<dd>Hello</dd>'))
+    const ast = compiler('<dd class="foo">Hello</dd>')
+    expect(ast).toMatchInlineSnapshot(`
+      <dd
+        className="foo"
+      >
+        Hello
+      </dd>
+    `)
 
+    render(ast)
     expect(root.innerHTML).toMatchInlineSnapshot(`
-      <dd>
+      <dd class="foo">
         Hello
       </dd>
     `)

index.tsx

@@ -89,7 +89,6 @@ const ATTRIBUTE_TO_JSX_PROP_MAP = [
   'cellPadding',
   'cellSpacing',
   'charSet',
-  'className',
   'classId',
   'colSpan',
   'contentEditable',
@@ -126,7 +125,7 @@ const ATTRIBUTE_TO_JSX_PROP_MAP = [
     obj[x.toLowerCase()] = x
     return obj
   },
-  { for: 'htmlFor' }
+  { class: 'className', for: 'htmlFor' }
 )
 
 const namedCodesToUnicode = {