[BUG] PHP Generator does not work with deepObject get params

Created by: nadar

Description

The PHP SDK generator does not support deepObject parameters. This is mainly because of \GuzzleHttp\Psr7\Query::build(). When using a nested object build() will return an PHP notice and the object will be added as ?filter=Array instead of expected: ?filter[foo]=bar

PHP Notice:  Array to string conversion in /vendor/guzzlehttp/psr7/src/Query.php on line 104
string(41) "?filter=Array"

openapi-generator version

latest dev-master

OpenAPI declaration file

An example which uses deepObject

- name: filter
  in: query
  description: Allows you to pass filtering options any attribute. `?filter[id]=123` or `?filter[author][like]=John`. Example of accessing nested array values `?filter[tags.alias]=food`
  required: false
  style: deepObject
  explode: true
  schema:
    type: object

Suggest a fix

What is the reason to use \GuzzleHttp\Psr7\Query::build? Because with php's http_build_query the code would work.

Remove the use of Query::build and replace with http_build_query from php, see https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/php/api.mustache#L671

If this is an accepted solution, i would like to provide a PR, maybe this could be at least introduced as an option... If there is something i miss, or there is a possibility to correctly build get URLs with deep object, please let me know. (Maybe i can also help document how to use deepObject) with the PHP SDK Generator.

Thanks for all the amazing work!