Use a better way to alter the page dictionary

[vbuch]

The old way fails with pdfkit 0.9.0. The new one works with both 0.8.3 and 0.9.0

[coveralls]

Pull Request Test Coverage Report for Build 125

• 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 100.0%

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Totals
Change from base Build 122:	0.0%
Covered Lines:	113
Relevant Lines:	113

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💛 - Coveralls

[vbuch]

@maldimirov, review