Don't tear down the ftp connection if the maximum filesize was exceeded

and added tests 290 and 291 to check.

tests/data/test291

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+<testcase>
+<info>
+<keywords>
+FTP
+RETR
+--max-filesize
+</keywords>
+</info>
+# Server-side
+<reply>
+<data>
+data
+    to
+      see
+that FTP
+works
+  so does it?
+</data>
+</reply>
+
+# Client-side
+<client>
+<server>
+ftp
+</server>
+ <name>
+FTP download maximum filesize not exceeded
+ </name>
+ <command>
+ftp://%HOSTIP:%FTPPORT/291 --max-filesize 100
+</command>
+</client>
+
+# Verify data after the test has been "shot"
+<verify>
+<protocol>
+USER anonymous
+PASS ftp@example.com
+PWD
+EPSV
+TYPE I
+SIZE 291
+RETR 291
+QUIT
+</protocol>
+</verify>
+</testcase>