gsutil cp -Z always force adds `Cache-control: no-transform` and `Content-Encoding: gzip`. Breaks http protocol

Curl client should be able to receive the unzipped version, but GCS always returns content-encoding gzip. This breaks HTTP/1.1 protocol since the client didn't give `"Accept-Encoding: gzip, deflate, br"` header.

```
$ gsutil -m -h "Cache-Control: public,max-age=31536000" cp -Z foo.txt gs://somebucket/foo.txt
Copying file://foo.txt [Content-Type=text/plain]...
- [1/1 files][ 42.0 B/ 12.0 B] 100% Done
Operation completed over 1 objects/12.0 B.

$ curl -v somebucket.io/foo.txt
> GET /foo1.txt HTTP/1.1
> User-Agent: curl/7.37.0
> Host: somebucket.io
> Accept: */*
>
< HTTP/1.1 200 OK
< X-GUploader-UploadID: ...
< Date: Thu, 19 Oct 2017 18:04:05 GMT
< Expires: Fri, 19 Oct 2018 18:04:05 GMT
< Last-Modified: Thu, 19 Oct 2017 18:03:47 GMT
< ETag: "c35fdf2f0c2dcadc46333b0709c87e64"
< x-goog-generation: 1508436227151587
< x-goog-metageneration: 1
< x-goog-stored-content-encoding: gzip
< x-goog-stored-content-length: 42
< Content-Type: text/plain
< Content-Encoding: gzip
< x-goog-hash: crc32c=V/9tDw==
< x-goog-hash: md5=w1/fLwwtytxGMzsHCch+ZA==
< x-goog-storage-class: MULTI_REGIONAL
< Accept-Ranges: bytes
< Content-Length: 42
< Access-Control-Allow-Origin: *
* Server UploadServer is not blacklisted
< Server: UploadServer
< Age: 2681
< Cache-Control: public,max-age=31536000,no-transform
< ��Y�tmpG1oc6S�H���W(�/�I���9�
```

Seems to be happening here

https://github.com/GoogleCloudPlatform/gsutil/blob/e8154bab37ad896b1e1ab01f452ac3284c7051d4/gslib/copy_helper.py#L1741-L1759