[Flight Fixture] Fix proxying with compression

[sebmarkbage]

We're decompressing and then writing and recompressing in the proxy. This causes it to stall if buffered because .pipe() doesn't force flush automatically.

[react-sizebot]

Comparing: ef8bdbe...048ac72

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	=	156.70 kB	156.70 kB	=	49.58 kB	49.58 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	158.70 kB	158.70 kB	=	50.25 kB	50.25 kB
facebook-www/ReactDOM-prod.classic.js	=	538.92 kB	538.92 kB	=	95.86 kB	95.86 kB
facebook-www/ReactDOM-prod.modern.js	=	522.78 kB	522.78 kB	=	93.55 kB	93.55 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 048ac72