Fix Net::Protocol::BufferedIO#write when sending large multi-byte string

	This commit should fix Net::Protocol::BufferedIO#write when sending
	large multi-byte string like following example.

	```
	$ ruby -rnet/http -rjson -v -e "Net::HTTP.post(URI('http://httpbin.org/post'), { text: '?'*100_000 }.to_json, 'Content-Type' => 'application/json')"
	ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]
	Traceback (most recent call last):
	        19: from -e:1:in `<main>'
	        18: from lib/ruby/2.6.0/net/http.rb:500:in `post'
	        17: from lib/ruby/2.6.0/net/http.rb:605:in `start'
	        16: from lib/ruby/2.6.0/net/http.rb:920:in `start'
	        15: from lib/ruby/2.6.0/net/http.rb:502:in `block in post'
	        14: from lib/ruby/2.6.0/net/http.rb:1281:in `post'
	        13: from lib/ruby/2.6.0/net/http.rb:1493:in `send_entity'
	        12: from lib/ruby/2.6.0/net/http.rb:1479:in `request'
	        11: from lib/ruby/2.6.0/net/http.rb:1506:in `transport_request'
	        10: from lib/ruby/2.6.0/net/http.rb:1506:in `catch'
	         9: from lib/ruby/2.6.0/net/http.rb:1507:in `block in transport_request'
	         8: from lib/ruby/2.6.0/net/http/generic_request.rb:123:in `exec'
	         7: from lib/ruby/2.6.0/net/http/generic_request.rb:189:in `send_request_with_body'
	         6: from lib/ruby/2.6.0/net/protocol.rb:247:in `write'
	         5: from lib/ruby/2.6.0/net/protocol.rb:265:in `writing'
	         4: from lib/ruby/2.6.0/net/protocol.rb:248:in `block in write'
	         3: from lib/ruby/2.6.0/net/protocol.rb:275:in `write0'
	         2: from lib/ruby/2.6.0/net/protocol.rb:275:in `each_with_index'
	         1: from lib/ruby/2.6.0/net/protocol.rb:275:in `each'
	lib/ruby/2.6.0/net/protocol.rb:280:in `block in write0': undefined method `bytesize' for nil:NilClass (NoMethodError)
	```

	[Fix GH-2058]

	From: Eito Katagiri <eitoball@gmail.com>


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66799 b2dd03c8-39d4-4d8f-98ff-823fe69b080e