perf($interpolate): optimize value stringification

previously we stringified numbers via toJson which was expensive, I optimized
the code so that toJson is invoked only if really necessary

Closes #7501

src/ng/interpolate.js

@@ -178,10 +178,24 @@ function $InterpolateProvider() {
                 } else {
                   part = $sce.valueOf(part);
                 }
-                if (part === null || isUndefined(part)) {
+                if (part == null) { // null || undefined
                   part = '';
-                } else if (typeof part != 'string') {
-                  part = toJson(part);
+                } else {
+                  switch (typeof part) {
+                    case 'string':
+                    {
+                      break;
+                    }
+                    case 'number':
+                    {
+                      part = '' + part;
+                      break;
+                    }
+                    default:
+                    {
+                      part = toJson(part);
+                    }
+                  }
                 }
               }
               concat[i] = part;