Force strings from stream for the infer strings strategy

[roll]

• fixes Issue with Infer Strategy/Cast Strategy and xlsx files BCODMO/frictionless-usecases#27

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@akariv
Without this change in the load processor, the added test fails which doesn't seem the correct behavior. I also had to update the strategies test. Please take a look at the issue above for more info about where it popped up.

WDYT?

[coveralls]

Pull Request Test Coverage Report for Build 401

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 84.623%

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Totals
Change from base Build 397:	0.02%
Covered Lines:	1651
Relevant Lines:	1951

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💛 - Coveralls

[akariv]

Yes, this makes sense.

Thanks!