blob/gcsblob: Ensure driver sets Content-Type auto-detection properly

#3371 made it possible to
disable Go Cloud's Content-Type auto-detection via the
`DisableContentTypeDetection` option. However, the Google Cloud
Storage (GCS) driver still performed auto-detection even if this
option were enabled, so it was previously not possible to keep
`Content-Type` blank. This commit adds the
`DisableContentTypeDetection` option to the the driver and passes
along the value to make it possible to keep Content-Type blank in the
GCS object metadata. This enables the use of the
`Response-Content-Type` override in a signed URL.

This commit needs cloud.google.com/go/storage v1.39
(googleapis/google-cloud-go#9431).

blob/blob.go

@@ -1083,14 +1083,15 @@ func (b *Bucket) NewWriter(ctx context.Context, key string, opts *WriterOptions)
 		opts = &WriterOptions{}
 	}
 	dopts := &driver.WriterOptions{
-		CacheControl:       opts.CacheControl,
-		ContentDisposition: opts.ContentDisposition,
-		ContentEncoding:    opts.ContentEncoding,
-		ContentLanguage:    opts.ContentLanguage,
-		ContentMD5:         opts.ContentMD5,
-		BufferSize:         opts.BufferSize,
-		MaxConcurrency:     opts.MaxConcurrency,
-		BeforeWrite:        opts.BeforeWrite,
+		CacheControl:                opts.CacheControl,
+		ContentDisposition:          opts.ContentDisposition,
+		ContentEncoding:             opts.ContentEncoding,
+		ContentLanguage:             opts.ContentLanguage,
+		ContentMD5:                  opts.ContentMD5,
+		BufferSize:                  opts.BufferSize,
+		MaxConcurrency:              opts.MaxConcurrency,
+		BeforeWrite:                 opts.BeforeWrite,
+		DisableContentTypeDetection: opts.DisableContentTypeDetection,
 	}
 	if len(opts.Metadata) > 0 {
 		// Services are inconsistent, but at least some treat keys

blob/driver/driver.go

@@ -100,6 +100,10 @@ type WriterOptions struct {
 	// Metadata holds key/value strings to be associated with the blob.
 	// Keys are guaranteed to be non-empty and lowercased.
 	Metadata map[string]string
+	// When true, the driver should attempt to disable any automatic
+	// content-type detection that the provider applies on writes with an
+	// empty ContentType.
+	DisableContentTypeDetection bool
 	// BeforeWrite is a callback that must be called exactly once before
 	// any data is written, unless NewTypedWriter returns an error, in
 	// which case it should not be called.

blob/gcsblob/gcsblob.go

@@ -628,6 +628,7 @@ func (b *bucket) NewTypedWriter(ctx context.Context, key string, contentType str
 		w.ChunkSize = bufferSize(opts.BufferSize)
 		w.Metadata = opts.Metadata
 		w.MD5 = opts.ContentMD5
+		w.ForceEmptyContentType = opts.DisableContentTypeDetection
 		return w
 	}