Fix: Support disabling DASH viewer in DASH-supported environment (#140)

Right now, if a user with a DASH-compatible browser disables the DASH viewer, then they cannot view any videos since the current X-Rep-Hints scheme hints to return either dash or mp4 reps, but not both. To fix this, this change updates the X-Rep-Hints sent to only send the mp4 (no filmstrip) when either the browser doesn't support DASH or if the DASH viewer is disabled.

src/lib/Preview.js

@@ -1109,7 +1109,9 @@ const PREVIEW_LOCATION = findScriptLocation(PREVIEW_SCRIPT_NAME, document.curren
      * @return {Object} Headers
      */
     getRequestHeaders(token) {
-        const videoHint = Browser.canPlayDash() ? X_REP_HINT_VIDEO_DASH : X_REP_HINT_VIDEO_MP4;
+        const videoHint = Browser.canPlayDash() && !this.disabledViewers.Dash
+            ? X_REP_HINT_VIDEO_DASH
+            : X_REP_HINT_VIDEO_MP4;
         const headers = {
             'X-Rep-Hints': `${X_REP_HINT_BASE}${X_REP_HINT_DOC_THUMBNAIL}${X_REP_HINT_IMAGE}${videoHint}`
         };

src/lib/__tests__/Preview-test.js

@@ -1628,6 +1628,15 @@ describe('lib/Preview', () => {
             preview.getRequestHeaders();
             expect(stubs.getHeaders).to.be.calledWith(stubs.headers, 'previewtoken', 'link', 'Passw0rd!');
         });
+
+        it('should not add dash hints if the browser supports dash but dash is disabled', () => {
+            stubs.canPlayDash.returns(true);
+            preview.disabledViewers.Dash = 1;
+            stubs.headers['X-Rep-Hints'] += '[mp4]';
+
+            preview.getRequestHeaders();
+            expect(stubs.getHeaders).to.be.calledWith(stubs.headers, 'previewtoken', 'link', 'Passw0rd!');
+        });
     });
 
     describe('prefetchNextFiles()', () => {