Add support for specifying any role on MenuItem.

Ran into a case where there is a readonly list displayed by a dropdown and the `<li />` role should be "reset" to `listitem`.

Author: cdeutsch

src/MenuItem.jsx

@@ -150,7 +150,7 @@ export class MenuItem extends React.Component {
       title: props.title,
       className,
       // set to menuitem by default
-      role: 'menuitem',
+      role: props.role || 'menuitem',
       'aria-disabled': props.disabled,
     };
 

tests/MenuItem.spec.js

@@ -129,7 +129,13 @@ describe('MenuItem', () => {
       expect(wrapper.render()).toMatchSnapshot();
     });
 
-    it('should set specific role', () => {
+    it('should set role to listitem', () => {
+      const wrapper = shallow(<NakedMenuItem role="listitem">test</NakedMenuItem>);
+
+      expect(wrapper.render()).toMatchSnapshot();
+    });
+
+    it('should set role to option', () => {
       const wrapper = shallow(<NakedMenuItem role="option">test</NakedMenuItem>);
 
       expect(wrapper.render()).toMatchSnapshot();

tests/__snapshots__/MenuItem.spec.js.snap

@@ -1,5 +1,14 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`MenuItem overwrite default role should set role to listitem 1`] = `
+<li
+  class="undefined-item"
+  role="listitem"
+>
+  test
+</li>
+`;
+
 exports[`MenuItem overwrite default role should set role to none if none 1`] = `
 <li
   class="undefined-item"
@@ -18,7 +27,7 @@ exports[`MenuItem overwrite default role should set role to none if null 1`] = `
 </li>
 `;
 
-exports[`MenuItem overwrite default role should set specific role 1`] = `
+exports[`MenuItem overwrite default role should set role to option 1`] = `
 <li
   class="undefined-item"
   role="option"