Use encoding from options on read.

Otherwise the hash is of the utf-8 interpretation of a binary file. For example:
```
$ openssl sha1 test/fixtures/file.png
SHA1(test/fixtures/file.png)= da63c1ef4358634812562d3732bad2a6941cd9ae
```
Is different from the test hash (a0539763). 

### Use case

I compare the hash w/ E-Tag in Amazon, which fails without this patch.