tests: remove superflous WriteHeader call from buffer tests

[adamvduke]

When running the tests for the buffer package with the -v flag, there is a log message that the call to WriteHeader from the TestBuffer_GRPC_OKResponse test is superfluous.

Before:

$ go test -v ./buffer -count=1 -run TestBuffer_GRPC_OKResponse
=== RUN   TestBuffer_GRPC_OKResponse
2024/12/16 11:55:54 http: superfluous response.WriteHeader call from github.com/vulcand/oxy/v2/buffer.TestBuffer_GRPC_OKResponse.func1 (buffer_test.go:420)
--- PASS: TestBuffer_GRPC_OKResponse (0.00s)
PASS
ok  	github.com/vulcand/oxy/v2/buffer	0.608s

After:

$ go test -v ./buffer -count=1 -run TestBuffer_GRPC_OKResponse
=== RUN   TestBuffer_GRPC_OKResponse
--- PASS: TestBuffer_GRPC_OKResponse (0.00s)
PASS
ok  	github.com/vulcand/oxy/v2/buffer	0.599s

[ldez]

LGTM