Metadata header containing accents

Hello

Running the following code makes node receive a 403 response from S3 server. By just replacing the accented character on custom headers (á) by an ASCII char (a) gives a successful response (200). Am I doing something wrong?

```
var knox = require('knox');

var client = knox.createClient({
key: '***',
secret: '***',
bucket: '***'
});

client.putFile(
'./my-file.jpg',
'/my-hash',
{
'x-amz-acl': 'public-read',
'Content-Type': 'image/jpeg',
'x-amz-meta-accent-test': 'á'
},
function (err, res) {
console.log(res.statusCode);
console.log('Err', err);
res.on('data', function (data) {
console.log(data.toString());
})
});
```

**Response (ommiting real data):**

```
403
Err null

SignatureDoesNotMatchThe request signature we calculated does not match the signature you provided. Check your key and signing method.............PUT

image/jpeg
Wed, 02 Apr 2014 22:25:47 GMT
x-amz-acl:public-read
x-amz-meta-accent-test:á
/my-hash...
```

Start by reproducing the client.putFile example with the accented x-amz-meta-accent-test header and compare it with the ASCII version. Read the request-signing path and use the S3 response's StringToSign to trace the differing header bytes. Done means the accented metadata request is accepted with a matching signature and a 200 response.

Written by the indexing model from the issue text.