Improve test coverage

[mddelk]

Was digging into thruster's source, figured I'd contribute a bit while I was there.

Thanks for this project!

---

Viewing coverage locally like so:

go test -v -coverprofile cover.out ./... && go tool cover -html cover.out -o cover.html && open cover.html

internal/config.go

Before:

After: 100%

internal/proxy_handler.go

Before:

After: 100%

internal/upstream_process.go

Before:

After: 100%

Total coverage changes

Before: (77.2%)

$ go test -coverprofile cover.out ./... && go tool cover -func=cover.out|rg total
        github.com/basecamp/thruster/cmd/thrust         coverage: 0.0% of statements
ok      github.com/basecamp/thruster/internal   0.022s  coverage: 78.7% of statements
total:                                                                  (statements)                    77.2%

After: (79.6%)

go test -coverprofile cover.out ./... && go tool cover -func=cover.out|rg total
        github.com/basecamp/thruster/cmd/thrust         coverage: 0.0% of statements
ok      github.com/basecamp/thruster/internal   0.024s  coverage: 81.1% of statements
total:                                                                  (statements)                    79.6%