fix: Fix parsing downloadFileCount property for BoxZipDownloadStatus

[BruceBlanchard]

API call returns "downloaded_file_count" not "download_file_count".. As such object is not representing the correct download count.

[CLAassistant]

All committers have signed the CLA.

[lukaszsocha2]

@BruceBlanchard thanks for creating this PR. Please accept Contributor License Agreement so that we can merge your pr.
Best,
@lukaszsocha2

[coveralls]

Pull Request Test Coverage Report for Build #4599

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.004%) to 72.062%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/com/box/sdk/BoxZipDownloadStatus.java	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/box/sdk/BoxFileUploadSession.java	1	64.67%
src/main/java/com/box/sdk/EventStream.java	1	86.46%
src/main/java/com/box/sdk/MetadataQuery.java	1	92.59%

Totals
Change from base Build #4597:	0.004%
Covered Lines:	8223
Relevant Lines:	11411

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build #4601

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.004%) to 72.062%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/com/box/sdk/BoxZipDownloadStatus.java	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/box/sdk/BoxFileUploadSession.java	1	64.67%
src/main/java/com/box/sdk/EventStream.java	1	86.46%
src/main/java/com/box/sdk/MetadataQuery.java	1	92.59%

Totals
Change from base Build #4597:	0.004%
Covered Lines:	8223
Relevant Lines:	11411

---

💛 - Coveralls