Use super instead of copy-and-paste in `Net::WebMockNetBufferedIO#i…

…nitialize`

The body of `Net::WebMockNetBufferedIO#initialize` is a code clone from `Net::BufferedIO#initialize`.
(BTW, it seems that `@continue_timeout = continue_timeout` lacks.)

It is difficult to maintain such a code clone.  Instead, it would be better to use `super` if possible.  This PR also should fix #786 gracefully.

lib/webmock/http_lib_adapters/net_http.rb

@@ -256,21 +256,18 @@ def readuntil(*args)
 module Net  #:nodoc: all
 
   class WebMockNetBufferedIO < BufferedIO
-    def initialize(io, read_timeout: 60, write_timeout: 60, continue_timeout: nil, debug_output: nil)
-      @read_timeout = read_timeout
-      @write_timeout = write_timeout
-      @rbuf = ''.dup
-      @debug_output = debug_output
-
-      @io = case io
+    def initialize(io, **)
+      io = case io
       when Socket, OpenSSL::SSL::SSLSocket, IO
         io
       when StringIO
         PatchedStringIO.new(io.string)
       when String
         PatchedStringIO.new(io)
       end
-      raise "Unable to create local socket" unless @io
+      raise "Unable to create local socket" unless io
+
+      super
     end
 
     if RUBY_VERSION >= '2.6.0'