fix(parser): Don't override position for implied opening tags

[fb55]

See posthtml/posthtml-parser#80 (comment)

[coveralls]

Pull Request Test Coverage Report for Build 1158871700

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 98.225%

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Totals
Change from base Build 1158694687:	0.004%
Covered Lines:	941
Relevant Lines:	958

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💛 - Coveralls