Add tests for `RenderMime.getRenderer()`

src/rendermime/index.ts

@@ -195,6 +195,8 @@ class RenderMime {
    * Get a renderer by mimetype.
    *
    * @param mimetype - The mimetype of the renderer.
+   *
+   * @returns The renderer for the given mimetype, or undefined if the mimetype is unknown.
    */
   getRenderer(mimetype: string): RenderMime.IRenderer {
     return this._renderers[mimetype];

test/src/rendermime/rendermime.spec.ts

@@ -217,6 +217,20 @@ describe('rendermime/index', () => {
 
     });
 
+    describe('#getRenderer()', () => {
+
+      it('should get a renderer by mimetype', () => {
+        let r = defaultRenderMime();
+        expect(r.getRenderer('text/plain')).to.be.a(TextRenderer);
+      });
+
+      it('should return undefined for missing mimetype', () => {
+        let r = defaultRenderMime();
+        expect(r.getRenderer('foo/bar')).to.be(undefined);
+      });
+
+    });
+
     describe('#mimetypes()', () => {
 
       it('should get the ordered list of mimetypes', () => {