core: add x-content-type-options: nosniff (#2730)
core: add x-content-type-options: nosniff (#2730)

This prevents browsers from guessing correct mimetype and make it follow
the content-type given by the server.

There was no functional regression from the change.
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard