feat(storage): make it possible to disable Content-Type sniffing

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).

Previously it wasn't possible to disable this auto-detection, even
though `googleapi.MediaOptions` provides a `ForceEmptyContentType`
option (https://github.com/googleapis/google-api-go-client/blob/v0.165.0/googleapi/googleapi.go#L283).

We enable this by adding a `Writer` option to set this value.

Closes googleapis#9430

storage/client.go

@@ -254,6 +254,9 @@ type openWriterParams struct {
 	// attrs - see `Writer.ObjectAttrs`.
 	// Required.
 	attrs *ObjectAttrs
+	// forceEmptyContentType - Disables auto-detect of Content-Type
+	// Optional.
+	forceEmptyContentType bool
 	// conds - see `Writer.o.conds`.
 	// Optional.
 	conds *Conditions

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 != "" || params.forceEmptyContentType {
 		mediaOpts = append(mediaOpts, googleapi.ContentType(c))
 	}
 	if params.chunkRetryDeadline != 0 {

storage/writer.go

@@ -88,6 +88,11 @@ type Writer struct {
 	// cancellation.
 	ChunkRetryDeadline time.Duration
 
+	// ForceEmptyContentType is an optional parameter that is used to disable
+	// auto-detection of Content-Type. By default, if a blank Content-Type
+	// is provided, then gax.DetermineContentType is called to sniff the type.
+	ForceEmptyContentType bool
+
 	// ProgressFunc can be used to monitor the progress of a large write
 	// operation. If ProgressFunc is not nil and writing requires multiple
 	// calls to the underlying service (see
@@ -180,18 +185,19 @@ func (w *Writer) openWriter() (err error) {
 	isIdempotent := w.o.conds != nil && (w.o.conds.GenerationMatch >= 0 || w.o.conds.DoesNotExist == true)
 	opts := makeStorageOpts(isIdempotent, w.o.retry, w.o.userProject)
 	params := &openWriterParams{
-		ctx:                w.ctx,
-		chunkSize:          w.ChunkSize,
-		chunkRetryDeadline: w.ChunkRetryDeadline,
-		bucket:             w.o.bucket,
-		attrs:              &w.ObjectAttrs,
-		conds:              w.o.conds,
-		encryptionKey:      w.o.encryptionKey,
-		sendCRC32C:         w.SendCRC32C,
-		donec:              w.donec,
-		setError:           w.error,
-		progress:           w.progress,
-		setObj:             func(o *ObjectAttrs) { w.obj = o },
+		ctx:                   w.ctx,
+		chunkSize:             w.ChunkSize,
+		chunkRetryDeadline:    w.ChunkRetryDeadline,
+		bucket:                w.o.bucket,
+		attrs:                 &w.ObjectAttrs,
+		conds:                 w.o.conds,
+		encryptionKey:         w.o.encryptionKey,
+		sendCRC32C:            w.SendCRC32C,
+		donec:                 w.donec,
+		setError:              w.error,
+		progress:              w.progress,
+		setObj:                func(o *ObjectAttrs) { w.obj = o },
+		forceEmptyContentType: w.ForceEmptyContentType,
 	}
 	if err := w.ctx.Err(); err != nil {
 		return err // short-circuit