Implemented `deduplicate_headers` flag for `load`

[roll]

• connects https://github.com/BCODMO/frictionless-usecases/issues/15

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It solves the problem with duplicate headers. There are a few questions:

• should we raise an error if there are duplicate headers and deduplicate_headers=False (default)
• should we have the flag at all or deduplication should be default behavior

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@akariv @cschloer

[coveralls]

Pull Request Test Coverage Report for Build 353

• 17 of 17 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 84.607%

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Totals
Change from base Build 346:	0.1%
Covered Lines:	1616
Relevant Lines:	1910

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💛 - Coveralls