Fix `BetterErrors::Middleware` for Content-Type that includes non MIME-Type parts

[r7kamura]

As the test case shows, there seemed to be a problem where Content-Type: application/json; charset=utf-8 did not work well, so I would like to fix this.

[coveralls]

Pull Request Test Coverage Report for Build 3493872322

• 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 96.811%

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Totals
Change from base Build 1215970624:	0.0%
Covered Lines:	516
Relevant Lines:	533

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💛 - Coveralls