[ruby-core:84873] [Ruby trunk Bug#14360] Regression CSV#open method for writing from Ruby 2.4.3 to 2.5.0

Issue #14360 has been reported by arturoherrero (Arturo Herrero).

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Bug #14360: Regression CSV#open method for writing from Ruby 2.4.3 to 2.5.0
https://bugs.ruby-lang.org/issues/14360

* Author: arturoherrero (Arturo Herrero)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-darwin17]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
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I have noticed a different behaviour between Ruby <= 2.4.3 and Ruby 2.5.0 for the `#open` method.

This issue is also open in its repository because it's not clear to me where is the appropriate place to open the issue, https://github.com/ruby/csv/issues/17

If you create an empty file for writing and you are not writing any line in that CSV file, Ruby <= 2.4.3 doesn't write anything (an empty file) but Ruby 2.5.0 writes the headers.

### Ruby <= 2.4.3
```shell
$ ruby -v
ruby 2.4.3p205 (2017-12-14 revision 61247) [x86_64-darwin17]
```

```ruby
$ irb
irb(main):001:0> require "csv"
=> true
irb(main):002:0> CSV.open("ruby-2.4.3.csv", "wb", headers: ["name", "surname"], write_headers: true) { }
=> nil
irb(main):003:0> `cat ruby-2.4.3.csv`
=> ""
```


### Ruby 2.5.0
```shell
$ ruby -v
ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-darwin17]
```

```ruby
$ irb
irb(main):001:0> require "csv"
=> true
irb(main):002:0> CSV.open("ruby-2.5.0.csv", "wb", headers: ["name", "surname"], write_headers: true) { }
=> nil
irb(main):003:0> `cat ruby-2.5.0.csv`
=> "name,surname\n"
```

---

In the examples, I'm using an empty block but in a real application probably you will have an `if` statement, something like this:

```ruby
CSV.open(...) do |csv|
  csv << "hello" if condition
end
```



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