Add limit argument to readline as well

The previous commit added a limit argument to #gets as defined by Ruby's
IO class. This does the same for readline since it also accepts a limit
argument in the IO class. Fortunately we simply forward these params to
gets.

Author: jstuckey

lib/net/sftp/operations/file.rb

@@ -115,8 +115,8 @@ def gets(sep_or_limit=$/, limit=Float::INFINITY)
 
     # Same as #gets, but raises EOFError if EOF is encountered before any
     # data could be read.
-    def readline(sep_string=$/)
-      line = gets(sep_string)
+    def readline(sep_or_limit=$/, limit=Float::INFINITY)
+      line = gets(sep_or_limit, limit)
       raise EOFError if line.nil?
       return line
     end