[Fizz/Flight] Fix strings are passed to destination

[shuding]

This ensures that all parts are converted to chunks before writing to the destination.

[sizebot]

Comparing: 90e5d36...2160d0e

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	130.19 kB	130.19 kB	=	41.42 kB	41.41 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	135.16 kB	135.16 kB	=	42.95 kB	42.95 kB
facebook-www/ReactDOM-prod.classic.js	=	419.90 kB	419.90 kB	=	77.34 kB	77.33 kB
facebook-www/ReactDOM-prod.modern.js	=	408.48 kB	408.48 kB	=	75.59 kB	75.59 kB
facebook-www/ReactDOMForked-prod.classic.js	=	419.90 kB	419.90 kB	=	77.34 kB	77.34 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 2160d0e