Don't guess the URL for files that end in .html

Fixes:
linkchecker ftp.html
failing looking for ftp://ftp.html

linkcheck/checker/__init__.py

@@ -35,6 +35,8 @@ def guess_url(url):
         a http respective ftp URL.
     @rtype: unicode
     """
+    if url.lower().endswith(".html") and "/" not in url:
+        return url
     if url.lower().startswith("www."):
         # syntactic sugar
         return "http://%s" % url

tests/checker/test_guess_url.py

@@ -0,0 +1,37 @@
+# Copyright (C) 2021 Chris Mayo
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+"""
+Test guess_url.
+"""
+
+import unittest
+from linkcheck.checker import guess_url
+
+
+class TestGuestUrl(unittest.TestCase):
+    """
+    Test guess_url.
+    """
+
+    def test_guess_url(self):
+        url = "www.example.com"
+        self.assertEqual(guess_url(url), f"http://{url}")
+        url = "ftp.example.com"
+        self.assertEqual(guess_url(url), f"ftp://{url}")
+        url = "ftp.html"
+        self.assertEqual(guess_url(url), url)
+        url = "www.html"
+        self.assertEqual(guess_url(url), url)