Add a test for a download without content-size

tests/end2end/features/downloads.feature

@@ -304,3 +304,11 @@ Feature: Downloading things from a website.
         And I run :prompt-accept (tmpdir)/subdir
         And I open data/downloads/download2.bin
         Then the download prompt should be shown with "{downloaddir}/download2.bin"
+
+    ## Other
+
+    Scenario: Download without a content-size
+        When I set storage -> prompt-download-directory to false
+        When I run :download http://localhost:(port)/custom/content-size
+        And I wait until the download is finished
+        Then the downloaded file content-size should exist

tests/end2end/fixtures/webserver_sub.py

@@ -84,6 +84,22 @@ def redirect_later_continue():
     return flask.Response(b'Continued redirect.')
 
 
+@app.route('/custom/content-size')
+def content_size():
+    """Send two bytes of data without a content-size."""
+
+    def generate_bytes():
+        yield b'*'
+        time.sleep(0.2)
+        yield b'*'
+
+    response = flask.Response(generate_bytes(), headers={
+        "Content-Type": "application/octet-stream",
+    })
+    response.status_code = 200
+    return response
+
+
 @app.after_request
 def log_request(response):
     """Log a webserver request."""