fix: class cast exception when uploading large file

[mhagmajer]

Closes #1173

[coveralls]

Pull Request Test Coverage Report for Build #2995

• 13 of 13 (100.0%) changed or added relevant lines in 3 files are covered.
• 5 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.05%) to 71.273%

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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	87.5%
src/main/java/com/box/sdk/MetadataQuery.java	1	92.59%
src/main/java/com/box/sdk/BoxAPIResponse.java	2	84.92%

Totals
Change from base Build #2972:	0.05%
Covered Lines:	7123
Relevant Lines:	9994

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💛 - Coveralls