Browser link injecting into MVC action response

Imagine that you have the following action in MVC 5 controller:

We create simple object on a fly, serialize it to string and return. What you might actually get? Well:

So it looks like browser link decided to inject itself just before body ending tag.

Why did it do so? If we check response headers, we will see the following one:

So application decides to send html (instead of javascript) so browser link decides to inject itself. Solution is obvious: