How to insert html to document using javascript

I'm newbie with JavaScript, so I have a problem to insert html into document using javascript.

Code that trying to insert html into document:

function loadTaskPage(taskId){
    fetch("https://localhost:44321/api/Tasks/1")
    .then(function(response){
        return response.text();
    }).then(function(data){
        document.body.insertAdjacentHTML('beforeend', data);
    }).catch(function(error){
        alert(error);
    })
}

This code part I took from tutorial, source code of tutorial could be found in this link: http://ift.tt/2v7VhEu

If I will try to open this link https://localhost:44321/api/Tasks/1 in browser I recieve normally styled web page, but when I try to insert it to document, html code got escaped and don't display anything.

Inserted html looks like:

<div id="\"myModal\"" class="\"modal" fade\"="">...

The code below is bootstrap modal copied from code examples. As you see there appeared symbols \" that escapes quotes.

Response with html I reciew from my ASP.Net Web Api with header: text/html

How should I insert this html code into document using javascript?