fix contain and containMatchingElement to accept string values

before, a string was accepted via react-element-to-jsx-string.
As that is no longer being used in favor of a custom function,
a string should still be accepted.

src/reactNodeToString.js

@@ -20,6 +20,8 @@ function reactArrayToJSXString (nodes) {
 export default function reactNodeToString (node) {
   if (Array.isArray(node)) {
     return reactArrayToJSXString(node)
+  } else if (typeof node === 'string') {
+    return node
   } else {
     return reactElementToJSXString(node)
   }

test/contain.test.js

@@ -22,6 +22,7 @@ class Fixture extends React.Component {
             <User index={2} />
             <User index={3} />
           </li>
+          <li>Unknown User</li>
         </ul>
       </div>
     )
@@ -35,6 +36,7 @@ describe('#contain', () => {
     it('passes when the actual matches the expected', (wrapper) => {
       expect(wrapper).to.contain(<User index={1} />)
       expect(wrapper).to.contain(<User index={2} />)
+      expect(wrapper).to.contain('Unknown User')
     }, { render: false })
 
     it('passes negated when the actual does not match the expected', (wrapper) => {

test/containMatchingElement.test.js

@@ -20,6 +20,7 @@ class Fixture extends React.Component {
         <ul>
           <li><User index={1} name='John' /></li>
           <li><User index={2} name='Doe' /></li>
+          <li>Unknown User</li>
         </ul>
       </div>
     )
@@ -33,6 +34,7 @@ describe('#containMatchingElement', () => {
     it('passes when the actual matches the expected', (wrapper) => {
       expect(wrapper).to.containMatchingElement(<User name='John' />)
       expect(wrapper).to.containMatchingElement(<User name='Doe' />)
+      expect(wrapper).to.containMatchingElement('Unknown User')
     }, { render: false })
 
     it('passes negated when the actual does not match the expected', (wrapper) => {