Fix ruby httpx test (#16765)

* fix setup of body params (may be nil)

* fix httpx adapter issues

treating DNS resolution errors as ConnectionFailed; removing multipart header set by the openapi-generated code, as it does not contain boundary, and interferes with the generation from httpx, which appropriately deals with mime-types already

samples/client/echo_api/ruby-httpx/lib/openapi_client/api_client.rb

@@ -65,7 +65,7 @@ module OpenapiClient
                 Net::HTTP::STATUS_CODES.fetch(response.status, "HTTP Error (#{response.status})")
       rescue HTTPX::TimeoutError
         fail ApiError.new('Connection timed out')
-      rescue HTTPX::ConnectionError
+      rescue HTTPX::ConnectionError, HTTPX::ResolveError
         fail ApiError.new('Connection failed')
       end
 
@@ -100,13 +100,11 @@ module OpenapiClient
         if config.debugging
           config.logger.debug "HTTP request body param ~BEGIN~\n#{req_body}\n~END~\n"
         end
-      else
-        body_params = {}
       end
       req_opts = {
-        :headers => HTTPX::Headers.new(header_params),
-        **body_params
+        :headers => HTTPX::Headers.new(header_params)
       }
+      req_opts.merge!(body_params) if body_params
       req_opts[:params] = query_params if query_params && !query_params.empty?
       session.request(http_method, url, **req_opts)
     end
@@ -121,14 +119,11 @@ module OpenapiClient
       # http form
       if header_params['Content-Type'] == 'application/x-www-form-urlencoded' ||
          header_params['Content-Type'] == 'multipart/form-data'
-        data = { form: form_params }
+        header_params.delete('Content-Type') # httpx takes care of this
+        { form: form_params }
       elsif body
-
-        data = body.is_a?(String) ? { body: body } : { json: body }
-      else
-        data = nil
+        body.is_a?(String) ? { body: body } : { json: body }
       end
-      data
     end
 
     def session