Remove usage of IO#nread

IO#wait, a method to obtain the number of bytes readable without
blocking, has been removed from io/wait.
ruby/io-wait@1decadc

When WEBrick is combined with io.wait >= 0.4.0, the following error
occurs on shutdown:

    ERROR NoMethodError: undefined method 'nread' for an instance of IO
        /(snip)/webrick-1.9.1/lib/webrick/server.rb:178:in 'block in WEBrick::GenericServer#start'

WEBrick uses IO#nread to efficiently consume the entire input stream
during shutdown. This patch removes the call to IO#nread, and instead
repeatedly take a fixed size of 8 bytes each iteration.

Author: osyoyu

lib/webrick/server.rb

@@ -173,9 +173,7 @@ def start(&block)
               if svrs = IO.select([sp, *@listeners])
                 if svrs[0].include? sp
                   # swallow shutdown pipe
-                  buf = String.new
-                  nil while String ===
-                            sp.read_nonblock([sp.nread, 8].max, buf, exception: false)
+                  nil while String === sp.read_nonblock(8, buf, exception: false)
                   break
                 end
                 svrs[0].each{|svr|