Fix #116: Static brotli overrides dynamic gzip

[adamburgess]

See #116 (comment)

Also adds two tests:

1. static .br files are served: this downloads a file that has a static .br and compares it to the static .br on disk.
   • This test passes without this patch, it is included to add a little testing to the static module.
2. dynamic gzip is used when .br doesn't exist: this downloads a file that does not have a static .br file, and ensures it is gzip compressed.
   • Without this patch, the test should fail.

[minac]

Hey 👋! Any news on when this can be merged?

[zhuizhuhaomeng]

@eustas Any news on when this can be merged?