[#268] client: Fix sequential PayloadPatch calls

* The flag 'firstPayloadPatch' keeps its state after first
  `PatchPayload` that make other calls incorrectly set patch
  ranges.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

client/object_patch_test.go

@@ -170,12 +170,11 @@ func TestObjectPatcher(t *testing.T) {
 			pk, _ := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
 
 			patcher := objectPatcher{
-				client:            &Client{},
-				stream:            m,
-				addr:              oidtest.Address(),
-				key:               pk,
-				maxChunkLen:       test.maxChunkLen,
-				firstPatchPayload: true,
+				client:      &Client{},
+				stream:      m,
+				addr:        oidtest.Address(),
+				key:         pk,
+				maxChunkLen: test.maxChunkLen,
 			}
 
 			success := patcher.PatchAttributes(context.Background(), nil, false)