Add SSLSocket#getbyte

Normal sockets respond to `getbyte`, so we should make SSLSocket respond
to `getbyte` as well.  This way we can substitute SSLSockets for regular
sockets.

lib/openssl/ssl.rb

@@ -416,6 +416,14 @@ def session
         nil
       end
 
+      # call-seq:
+      #   ssl.getbyte => 81
+      #
+      # Get the next 8bit byte from `ssl`
+      def getbyte
+        read(1).unpack1("C")
+      end
+
       private
 
       def using_anon_cipher?

test/openssl/test_ssl.rb

@@ -185,6 +185,24 @@ def test_sysread_and_syswrite
     }
   end
 
+  def test_getbyte
+    start_server { |port|
+      server_connect(port) { |ssl|
+        str = +("x" * 100 + "\n")
+        ssl.syswrite(str)
+        newstr = str.bytesize.times.map { |i|
+          ssl.getbyte
+        }.pack("C*")
+        assert_equal(str, newstr)
+
+        buf = String.new
+        ssl.syswrite(str)
+        assert_same buf, ssl.sysread(str.size, buf)
+        assert_equal(str, buf)
+      }
+    }
+  end
+
   def test_sync_close
     start_server do |port|
       begin