add ruby httpx echo api tests (#16729)

samples/client/echo_api/ruby-httpx/docs/HeaderApi.md

@@ -0,0 +1,78 @@
+# OpenapiClient::HeaderApi
+
+All URIs are relative to *http://localhost:3000*
+
+| Method | HTTP request | Description |
+| ------ | ------------ | ----------- |
+| [**test_header_integer_boolean_string**](HeaderApi.md#test_header_integer_boolean_string) | **GET** /header/integer/boolean/string | Test header parameter(s) |
+
+
+## test_header_integer_boolean_string
+
+> String test_header_integer_boolean_string(opts)
+
+Test header parameter(s)
+
+Test header parameter(s)
+
+### Examples
+
+```ruby
+require 'time'
+require 'openapi_client'
+
+api_instance = OpenapiClient::HeaderApi.new
+opts = {
+  integer_header: 56, # Integer | 
+  boolean_header: true, # Boolean | 
+  string_header: 'string_header_example' # String | 
+}
+
+begin
+  # Test header parameter(s)
+  result = api_instance.test_header_integer_boolean_string(opts)
+  p result
+rescue OpenapiClient::ApiError => e
+  puts "Error when calling HeaderApi->test_header_integer_boolean_string: #{e}"
+end
+```
+
+#### Using the test_header_integer_boolean_string_with_http_info variant
+
+This returns an Array which contains the response data, status code and headers.
+
+> <Array(String, Integer, Hash)> test_header_integer_boolean_string_with_http_info(opts)
+
+```ruby
+begin
+  # Test header parameter(s)
+  data, status_code, headers = api_instance.test_header_integer_boolean_string_with_http_info(opts)
+  p status_code # => 2xx
+  p headers # => { ... }
+  p data # => String
+rescue OpenapiClient::ApiError => e
+  puts "Error when calling HeaderApi->test_header_integer_boolean_string_with_http_info: #{e}"
+end
+```
+
+### Parameters
+
+| Name | Type | Description | Notes |
+| ---- | ---- | ----------- | ----- |
+| **integer_header** | **Integer** |  | [optional] |
+| **boolean_header** | **Boolean** |  | [optional] |
+| **string_header** | **String** |  | [optional] |
+
+### Return type
+
+**String**
+
+### Authorization
+
+No authorization required
+
+### HTTP request headers
+
+- **Content-Type**: Not defined
+- **Accept**: text/plain
+