Fix Call to undefined function GuzzleHttp\Psr7\modify_request()

[alcalyn]

I needed to do this change in order to prevent this error Call to undefined function GuzzleHttp\Psr7\modify_request().

This PR removes deprecated calls to guzzle functions.

I checked all the code, and this was the only missing call to GuzzleHttp\Psr7\ deprecated functions. I updated the unit tests.

[coveralls]

Pull Request Test Coverage Report for Build 709

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.196%

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Totals
Change from base Build 706:	0.0%
Covered Lines:	1669
Relevant Lines:	1735

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💛 - Coveralls

[haphan]

Thanks @alcalyn 🚀