Force client to use UTF-8 encoding by setting encoding in Content-Typ…

[stanogurnik]

Force client to use UTF-8 encoding by setting encoding in Content-Type header.

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[coveralls]

Coverage remained the same at 100.0% when pulling 18f53a8 on stanogurnik:set-utf-8-encoding into 6dcf37a on graphql:master.

[ghost]

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[leebyron]

Good find.