A search with SimilarImage raises an exception

# Issue report

## Brief description of issue

Try to search with SimilarImage using code like:

```
$search_params = new SearchParameters();
$search_params->setSimilarImage(true);

$request = new SearchFilesRequest();
$request->setLocale('en-US');
$request->setSearchParams($search_params);
$request->setSimilarImage("myimage.jpg");

$response = $client->searchFilesInitialize($request, $accessToken)->getNextResponse();
```

You will get a StockApi exception with the error message "Image File is not readable".

The exception is raised in HttpClient.php:92, where it checks if `$file` is a readable file. However, `$file` contains the binary content of the image file, not the image name. Just below this code, `$file` is used as the contents of a multipart upload (assuming it contains the content of the image). My guess is the `if (!is_readable($file)) { ... }` block is not needed and is introducing this bug.

## Proposed fix (if applicable)

Remove the `if (!is_readable($file)) { ... }`