improve performance of isUtf8MimeType

[Uzlopak]

before:

node benchmarks/isUtf8MimeType.js
isUtf8MimeType x 10,632,205 ops/sec ±1.69% (191 runs sampled)

after:

node benchmarks/isUtf8MimeType.js
isUtf8MimeType x 1,132,847,245 ops/sec ±0.15% (192 runs sampled)

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct