FileInputStream is not closed and the refFile cannot be deleted

[lijiajun05]

1、Springboot project. maven dependency：

io.swagger
swagger-parser
1.0.55

2、Issue location:
class: io.swagger.parser.util.RefUtils
method: readExternalRef()
line nums: 141, 152

3、Issue description:
Source code:result = IOUtils.toString(new FileInputStream(pathToUse.toFile()), "UTF-8");
The method IOUtils.toString() do not flush or close the stream. So new FileInputStream(pathToUse.toFile() is not closed and the file cannot be deleted.
In my project, the refFiles are temporary and need be deleted, then found this issue.
Thanks for replying to this issue.

4、Suggestion:
FileUtils.readFileToString() could be considered.

[frantuma]

fixed in #1721