[Zend\Http\Client] dupplicate header keys in prepareHeaders

in my mind I found a bug in the Http Component. In my gist I write a TestCase to show the error.

if we you a toString-Methode from ```\Zend\Http\Header\ContentType```

https://gist.github.com/ClemensSahs/6421612


The provenance from this issues is in the method ```\Zend\Http\Client::prepareHeaders``` here we use not the lowercase string for the header keys like the ```\Zend\Http\Header\*``` So we get this.

```php
// 1st ( store )
array(6) {
  ["Host"]=>
  string(14) "127.0.0.1:5984"
  ["Connection"]=>
  string(5) "close"
  ["Accept-Encoding"]=>
  string(13) "gzip, deflate"
  ["User-Agent"]=>
  string(16) "Zend\Http\Client"
  ["Content-Length"]=>
  int(13)
  ["content-type"]=>
  string(16) "application/json"
}

// 2nd ( get )
array(5) {
  ["Host"]=>
  string(14) "127.0.0.1:5984"
  ["Connection"]=>
  string(5) "close"
  ["Accept-Encoding"]=>
  string(13) "gzip, deflate"
  ["User-Agent"]=>
  string(16) "Zend\Http\Client"
  ["Content-Type"]=>
  string(16) "application/json"
}
// 3rd ( store )
array(7) {
  ["Host"]=>
  string(14) "127.0.0.1:5984"
  ["Connection"]=>
  string(5) "close"
  ["Accept-Encoding"]=>
  string(13) "gzip, deflate"
  ["User-Agent"]=>
  string(16) "Zend\Http\Client"
  ["Content-Type"]=>
  string(16) "application/json"
  ["Content-Length"]=>
  int(13)
  ["content-type"]=>
  string(16) "application/json"
}
```




best regards