Add multiple tag suppport

[jebentier]

This is doing a couple things to bring this gem more in alignment with the expectations of the Consul API:

1. It is generalizing the Faraday connection to always use FlatParamsEncoder. This is because Consul's API has already taken the stance that they do not expect multiple params to be specified using [] but instead by simply repeating the same query param multiple times.
2. Adding support to Diplomat::Health.service for specifying an array of tags when querying for a service's health checks

Fixes #212

[pierresouchay]

@jebentier Your Pull request has been merged and published into release 2.4.3

For your info: if you use Consul and needs to generate templates or UIs, you might also have a look at https://github.com/criteo/consul-templaterb/ (which is far more efficient and easy to hack than diplomat if you just need templates)