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: 'あ'*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/trunk@66582 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

0 files changed, 0 insertions, 0 deletions