Fix ETag handling with auto-encryption with CopyObject conditions (#7000)

minio-java tests were failing under multiple places when
auto encryption was turned on, handle all the cases properly

This PR fixes

 - CopyObject should decrypt ETag before it does if-match
 - CopyObject should not try to preserve metadata of source
   when rotating keys, unless explicitly asked by the user.
 - We should not try to decrypt Compressed object etag, the
   potential case was if user sets encryption headers along
   with compression enabled.

cmd/object-api-utils.go

@@ -503,6 +503,9 @@ func NewGetObjectReader(rs *HTTPRangeSpec, oi ObjectInfo, cleanUpFns ...func())
 				return nil, err
 			}
 
+			// Decrypt the ETag before top layer consumes this value.
+			oi.ETag = getDecryptedETag(h, oi, copySource)
+
 			// Apply the skipLen and limit on the
 			// decrypted stream
 			decReader = io.LimitReader(ioutil.NewSkipReader(decReader, skipLen), decRangeLength)