chore: upgrade net-http-persistent library to now version

Signed-off-by: Javier Criado Marcos <jcriadomarco@vmware.com>

Author: javiercri

fluent-plugin-datadog.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_runtime_dependency "fluentd", [">= 1", "< 2"]
-  spec.add_runtime_dependency "net-http-persistent", '~> 3.1'
+  spec.add_runtime_dependency "net-http-persistent", '~> 4.0.1'
 
   spec.add_development_dependency "bundler", "~> 2.1"
   spec.add_development_dependency "test-unit", '~> 3.1'