Fix FTP checker

In Python 2 StringIO could accept either Unicode or 8-bit strings.
Similar change made for HttpUrl:
06fdd78f9 ("Python3: fix TypeError in HttpUrl.read_content()",
2019-09-15)

Non-existent FtpUrl.max_size introduced in:
7b34be590 ("Introduce check plugins, use Python requests for http/s
connections, and some code cleanups and improvements.", 2014-03-01)

Additional self.direct() not added in:
f107092a8 ("Fix handling of user/password info in URLs.", 2012-06-10)

linkcheck/checker/ftpurl.py

@@ -18,7 +18,7 @@ Handle FTP links.
 """
 
 import ftplib
-from io import StringIO
+from io import BytesIO
 
 from .. import log, LOG_CHECK, LinkCheckerError, mimeutil
 from . import internpaturl, get_index_html
@@ -179,12 +179,12 @@ class FtpUrl(internpaturl.InternPatternUrl):
         else:
             # download file in BINARY mode
             ftpcmd = f"RETR {self.filename}"
-            buf = StringIO()
+            buf = BytesIO()
 
             def stor_data(s):
                 """Helper method storing given data"""
                 # limit the download size
-                if (buf.tell() + len(s)) > self.max_size:
+                if (buf.tell() + len(s)) > self.aggregate.config["maxfilesizedownload"]:
                     raise LinkCheckerError(_("FTP file size too large"))
                 buf.write(s)
 

tests/checker/test_ftp.py

@@ -33,6 +33,7 @@ class TestFtp(FtpServerTest):
             "real url %s" % url,
             "valid",
         ]
+        self.direct(url, resultlines)
         # ftp use/password
         user = "anonymous"
         passwd = "Ftp"