Ajax put upload SignatureDoesNotMatch bug solve

I want use ajax upload file to s3 with put method.

First, I generate s3 object write url use `aws-sdk`

```ruby
require 'aws-sdk'
AWS.config(
  access_key_id: ENV['AWS_ACCESS_KEY_ID'],
  secret_access_key: ENV['AWS_SECRET_ACCESS_KEY']
)
s3 = AWS::S3.new
bucket = s3.buckets[ENV['AWS_S3_BUCKET']]
obj = bucket.objects['tmp/file']
url = obj.url_for :write, expires: 3600
url.to_s
```

Second, i use ajax upload, use default options

the upload html: https://github.com/vkill/browser_upload_file_to_s3_demo/blob/master/public/upload_test.html

```javascript
$.ajax({
    type: 'put',
    url: url,
    dataType: 'xml',
    processData: false,
    data: file
});
```

the result like this

>OPTIONS https://dev-playab-net.s3.amazonaws.com/tmp/file?AWSAccessKeyId=AKIAI6GLQRTI5FAZMNRA&Expires=1364064748&Signature=7TYtdwb%2BSVK%2FTwPIok%2FCuvWbYTw%3D 200 (OK) 
>PUT https://dev-playab-net.s3.amazonaws.com/tmp/file?AWSAccessKeyId=AKIAI6GLQRTI5FAZMNRA&Expires=1364064748&Signature=7TYtdwb%2BSVK%2FTwPIok%2FCuvWbYTw%3D 403 (Forbidden) 

```xml
<?xml version="1.0" encoding="UTF-8"?>
<Error><Code>SignatureDoesNotMatch</Code><Message>The request signature we calculated does not match the signature you provided. Check your key and signing method.</Message><StringToSignBytes>50 55 54 0a 0a 61 70 70 6c 69 63 61 74 69 6f 6e 2f 78 2d 77 77 77 2d 66 6f 72 6d 2d 75 72 6c 65 6e 63 6f 64 65 64 3b 20 63 68 61 72 73 65 74 3d 55 54 46 2d 38 0a 31 33 36 34 30 36 34 37 34 38 0a 2f 64 65 76 2d 70 6c 61 79 61 62 2d 6e 65 74 2f 74 6d 70 2f 66 69 6c 65</StringToSignBytes><RequestId>A2659C0EAF2E0BDD</RequestId><HostId>hGIUqnKQDXVpRyl98OdSjWTlrIdQF2HVFITulrLby4WTuSiq7NeDtRiUudjkhVOh</HostId><SignatureProvided>7TYtdwb+SVK/TwPIok/CuvWbYTw=</SignatureProvided><StringToSign>PUT

application/x-www-form-urlencoded; charset=UTF-8
1364064748
/dev-playab-net/tmp/file</StringToSign><AWSAccessKeyId>AKIAI6GLQRTI5FAZMNRA</AWSAccessKeyId></Error> 
```

Third,  i set Content-Type header and use ajax upload

```javascript
$.ajax({
    type: 'put',
    url: url,
    dataType: 'xml',
    processData: false,
    data: file,
    headers: {
      'Content-Type': file.type
    }
});
```

the result like this

>OPTIONS https://dev-playab-net.s3.amazonaws.com/tmp/file?AWSAccessKeyId=AKIAI6GLQRTI5FAZMNRA&Expires=1364064748&Signature=7TYtdwb%2BSVK%2FTwPIok%2FCuvWbYTw%3D 200 (OK) 

>PUT https://dev-playab-net.s3.amazonaws.com/tmp/file?AWSAccessKeyId=AKIAI6GLQRTI5FAZMNRA&Expires=1364064748&Signature=7TYtdwb%2BSVK%2FTwPIok%2FCuvWbYTw%3D 403 (Forbidden)

```xml
<?xml version="1.0" encoding="UTF-8"?>
<Error><Code>SignatureDoesNotMatch</Code><Message>The request signature we calculated does not match the signature you provided. Check your key and signing method.</Message><StringToSignBytes>50 55 54 0a 0a 61 70 70 6c 69 63 61 74 69 6f 6e 2f 78 2d 72 75 62 79 0a 31 33 36 34 30 36 34 37 34 38 0a 2f 64 65 76 2d 70 6c 61 79 61 62 2d 6e 65 74 2f 74 6d 70 2f 66 69 6c 65</StringToSignBytes><RequestId>5C3072B338FDF1BE</RequestId><HostId>ZMaN/ccYiRE61ZOzbApu8iZaFyO/YTyGG7LfZDAtKZGNv3oX0S9dAOUBj/uHjR7p</HostId><SignatureProvided>7TYtdwb+SVK/TwPIok/CuvWbYTw=</SignatureProvided><StringToSign>PUT

application/x-ruby
1364064748
/dev-playab-net/tmp/file</StringToSign><AWSAccessKeyId>AKIAI6GLQRTI5FAZMNRA</AWSAccessKeyId></Error> 
```

So, i view source, https://github.com/aws/aws-sdk-ruby/blob/master/lib/aws/s3/s3_object.rb#L1340 and https://github.com/aws/aws-sdk-ruby/blob/master/lib/aws/s3/request.rb#L93 , discover `s3_object#string_to_sign` error,  missing 'content-md5' header and 'content-type' header.

I already solved it, please merge.

Thanks.