Change stub socket for Ruby 2.4 compat

Background:

1.  As of Ruby 2.2, calling #close on a socket that's already closed
    no longer raises an exception.

2.  As of Ruby 2.4, Net::HTTP no longer checks #closed? every time
    before calling #close on a socket, since there's no longer any
    danger it will raise. See:

    * r56795 (ruby/ruby@6394b63db9abe34234e7)
    * r56865 (ruby/ruby@f845a9ef76c0195254de)

FakeWeb's internal StubSocket has been short-circuiting those pre-2.4
checks by just always returning true when #closed? is checked,
preventing #close from ever being called.

Now that the checks are gone, StubSocket needs its own #close
method. This fixes a variety of NoMethodErrors on Ruby 2.4, but is
basically backwards-compatible with earlier implementations.

(When running on pre-Ruby 2.2, I suppose StubSocket should raise when
you call #close on an already-closed socket. I'll look into that, but
all our functional tests are passing on earlier Rubies at the moment.)

While we're at it, I changed #closed? to stop just always returning
true, to be closer to how the Ruby stdlib's sockets behave. This
doesn't appear to break anything but I'll revert that part if I find
anything in the relevant Net::HTTP code that conflicts.

Thanks for reporting and submitting patches: @voxik in #59,
@SamMolokanov in #60.

Closes #59.
References #57.

lib/fake_web/stub_socket.rb

@@ -9,7 +9,12 @@ def initialize(*args)
     end
 
     def closed?
-      @closed ||= true
+      @closed ||= false
+      @closed
+    end
+
+    def close
+      @closed = true
     end
 
     def readuntil(*args)

test/test_stub_socket_compatibility.rb

@@ -30,4 +30,22 @@ def test_stub_socket_only_responds_to_continue_timeout_under_193_or_later
     socket = http.instance_variable_get(:@socket)
     assert_equal RUBY_VERSION >= "1.9.3", socket.respond_to?(:continue_timeout=)
   end
+
+  def test_stub_socket_responds_to_close_and_always_returns_true
+    FakeWeb.register_uri(:get, "http://example.com", :status => [200, "OK"])
+    http = Net::HTTP.new("example.com", 80)
+    http.get("/")
+    socket = http.instance_variable_get(:@socket)
+    assert_equal socket.close, true
+  end
+
+  def test_stub_socket_tracks_closed_state
+    FakeWeb.register_uri(:get, "http://example.com", :status => [200, "OK"])
+    http = Net::HTTP.new("example.com", 80)
+    http.get("/")
+    socket = http.instance_variable_get(:@socket)
+    assert !socket.closed?
+    socket.close
+    assert socket.closed?
+  end
 end