Added the possibility to set a proxy URL for the client

[EtienneDepaulis]

This PR adds an attribute proxy_url to the FHIR::Client object. It helps set up a proxy URL.

For the RestClient strategy, it was possible to define it after setting the strategy, for the OAuth2::Client it was needed inside in order to provide this configuration before retrieving the token.

[arscan]

Thanks for another contribution!

We have been simply using the http_proxy and https_proxy environment variables to handle proxies, which get picked up by the libraries we are using. Is there a situation where this doesn't work?

[EtienneDepaulis]

Not a bad idea, I completely forgot about this feature :/ Thx for pointing it out :)

[EtienneDepaulis]

@arscan We identified an issue using the HTTPS_PROXY setting on Heroku : it looks like it is used during build time which prevents the app from being deployed :/
Therefore I would like to re-submit this PR as it can help avoid this kind of conflict on certain PaaS

[arscan]

Sure, since both RestClient and OAuthClient have a proxy attribute, it seems to make sense for this library to also. Would you consider naming it proxy just to be consistent with those other two libraries we use?

And I just accepted PR #105, which creates a new client to resolve external references. I assume we should have that client use the same proxy as this client. Could you pull in those changes here (or rebase) and add in this proxy to that?

And finally, could you update the README with a quick blurb about this (just near the end is fine).

Thanks!

[EtienneDepaulis]

@arscan I've updated the naming convention in favor of proxy and added basic info in the Readme (I also included the additional_headers which was missing in the documentation).

As for #105 I'm not certain about how to tackle this problem. What about other settings ?

[arscan]

Thanks! I think you forgot to rename proxy_url to proxy in a few places though, since a bunch of tests are failing now.

And I was just referring to these two lines:
https://github.com/fhir-crucible/fhir_client/blob/master/lib/fhir_client/ext/reference.rb#L62
https://github.com/fhir-crucible/fhir_client/blob/master/lib/fhir_client/ext/reference.rb#L72

To follow external references we have to create a new client (because it is a new FHIR endpoint), and I would think we would want to set the proxy correctly on those new clients, or is that not necessary given the way that RestClient is referenced?

Also, I'm a bit confused about how this is used. It doesn't seem like the proxy is set until you explicitly set an auth scheme (or set_no_auth)? Just doing client.proxy = 'http:https://xyz.com' won't propagate the change to RestClient or OAuthClient, until something like client.set_no_auth is called?

[EtienneDepaulis]

@arscan I fixed the typo, wrong copy/paste I guess :/

My PR only worked for authenticated clients. I changed this behavior and added a param on the initialize method and also updated the 2 clients mentioned in your previous comment.

However, I was wondering how the external approach works for authenticated clients as only the basic params are passed on.

[radamson]

Thanks for the contribution and updates!

[EtienneDepaulis]

Thx @radamson :)