storage: make it possible to disable Content-Type sniffing

[stanhu]

Is your feature request related to a problem? Please describe.

As described in google/go-cloud#3298 (comment), we want to disable automatic Content-Type detection when inserting an object to Google Cloud Storage (GCS).

To write a blob to GCS, the Go Cloud project calls httpStorageClient.OpenWriter, which calls this to insert the blob:

google-cloud-go/storage/http_client.go

Lines 907 to 911 in 2090651

	call := c.raw.Objects.Insert(params.bucket, rawObj).
	Media(pr, mediaOpts...).
	Projection("full").
	Context(params.ctx).
	Name(params.attrs.Name)

It appears that googleapis.MediaOptions has ForceEmptyContentType that can be used to disable this content sniffing: https://github.com/googleapis/google-api-go-client/blob/260fc1ef78c67de11cc5d654fcbe08edd8c01843/internal/gensupport/media.go#L152-L153

However, there's currently no way to set this value.

Describe the solution you'd like

Likely the easiest option would be to add an option to openWriterParams to enable ForceEmptyContentType.

[stanhu]

I've confirmed that this hack properly disables the sniffing, since googleapi.ContentType will set ForceEmptyContentType to be true if the value is blank (https://github.com/googleapis/google-api-go-client/blob/260fc1ef78c67de11cc5d654fcbe08edd8c01843/googleapi/googleapi.go#L230-L235):

diff --git a/storage/http_client.go b/storage/http_client.go
index a0f3c00a73..e3e0d761bb 100644
--- a/storage/http_client.go
+++ b/storage/http_client.go
@@ -885,7 +885,7 @@ func (c *httpStorageClient) OpenWriter(params *openWriterParams, opts ...storage
        mediaOpts := []googleapi.MediaOption{
                googleapi.ChunkSize(params.chunkSize),
        }
-       if c := attrs.ContentType; c != "" {
+       if c := attrs.ContentType; c != "" || true {
                mediaOpts = append(mediaOpts, googleapi.ContentType(c))
        }
        if params.chunkRetryDeadline != 0 {