Disable auto-decompression in GCS persistor

The GCS library automatically decompresses files with Content-encoding:
gzip. This is very useful, but the S3 library doesn't do that, so we
need to disable the feature in GCS so that it behaves the same way other
persistors behave.

libraries/object-persistor/src/GcsPersistor.js

@@ -102,7 +102,7 @@ module.exports = class GcsPersistor extends AbstractPersistor {
     const stream = this.storage
       .bucket(bucketName)
       .file(key)
-      .createReadStream(opts)
+      .createReadStream({ decompress: false, ...opts })
 
     // ingress to us from gcs
     const observer = new PersistorHelper.ObserverStream({

libraries/object-persistor/test/unit/GcsPersistorTests.js

@@ -160,6 +160,12 @@ describe('GcsPersistorTests', function () {
         expect(GcsFile.createReadStream).to.have.been.called
       })
 
+      it('disables automatic decompression', function () {
+        expect(GcsFile.createReadStream).to.have.been.calledWith({
+          decompress: false
+        })
+      })
+
       it('pipes the stream through the meter', function () {
         expect(ReadStream.pipe).to.have.been.calledWith(
           sinon.match.instanceOf(Transform)
@@ -183,6 +189,7 @@ describe('GcsPersistorTests', function () {
 
       it('passes the byte range on to GCS', function () {
         expect(GcsFile.createReadStream).to.have.been.calledWith({
+          decompress: false,
           start: 5,
           end: 10
         })