util: import http utilities from async-http-client

Merci Jean-François et Stéphane !

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
payment: add default implementation of HostedPaymentPageFormDescriptor

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
api: add convenience method PluginProperties#merge

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
test: use real locale when mocking account data

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
payment: add standard amount plugin property

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>