title: Use Fetch-Mock to return test Blob
body: <p>I have a fetch which receives a file from a server and I am trying to mock the fetch for my tests with <code>fetch-mock</code>.<br>
  Using this code I can mock the endpoint and place the blob in the body: </p>
  
  <pre><code>const blob = new Blob(['a', 'b', 'c', 'd']);
  fetchMock.once('*', {body: blob}, {sendAsJson: false});
  </code></pre>
  
  <p>The code that is being tested is:</p>
  
  <pre><code>fetch(url).then( ( response ) =&gt; {
              console.log(response);
              return response.blob();
          } ).then( ( blob ) =&gt; {
              console.log(blob);
          } )
  </code></pre>
  
  <p>I can see that the Blob is on the body of the request</p>
  
  <pre><code>Body {
      url: '/mock/url',
      status: 200,
      statusText: 'OK',
      headers: Headers { _headers: {} },
      ok: true,
      body: Blob {},
      bodyUsed: false,
      size: 0,
      timeout: 0,
      _raw: [],
      _abort: false }
  </code></pre>
  
  <p>But running the test throws the error:</p>
  
  <pre><code>    TypeError: response.blob is not a function
  </code></pre>
  
  <p>Running the code with a server returns a valid Blob into the final <code>.then</code>.</p>