Close socket when client sends EOF

[michal-kazmierczak]

As is:
When a client triggers EOF in server to close connection (in Ruby client ftp.close) the server doesn't handle this signal and forces the client to wait on read.

To be:
When EOF is received, close the socket immediately.

How to reproduce:

FakeFtp::Server.new(21212)
server.start

ftp_client = Net::FTP.new
ftp_client.connect('127.0.0.1', 21212)
ftp_client.login('user', 'password')
Benchmark.measure{ ftp_client.close }
=> #<Benchmark::Tms:0x00007fecfad73068
 @cstime=0.0,
 @cutime=0.0,
 @label="",
 @real=3.003513000003295,
 @stime=0.15119499999999997,
 @total=3.000451,
 @utime=2.849256>

...

Another run on this branch:

FakeFtp::Server.new(21212)
server.start

ftp_client = Net::FTP.new
ftp_client.connect('127.0.0.1', 21212)
ftp_client.login('user', 'password')
Benchmark.measure{ ftp_client.close }
=> #<Benchmark::Tms:0x00007fefe91c6688
 @cstime=0.0,
 @cutime=0.0,
 @label="",
 @real=0.0002390000008745119,
 @stime=8.900000000000574e-05,
 @total=0.0002629999999999022,
 @utime=0.00017399999999989646>

This is where the 3s of wait comes from https://github.com/ruby/ruby/blob/ruby_2_6/lib/net/ftp.rb#L1308

Furthermore, I suspect that this is the reason why integration and functional tests are disabled by default in this project. With this fix, the execution time shrinks significantly - from Finished in 21.24 seconds (files took 0.14215 seconds to load) to Finished in 0.21783 seconds (files took 0.14647 seconds to load)