Currency it fixes

So I realized that we were still using POST requests in the integration tests for all the endpoints.. i should possibly add extra tests for doing those as well, but it seems correct at least to at least rather make them match the docs.

I also changed the currency endpoints to use /currency/<provider> instead of the query params.

Finally I also added an integration test for landsbankinn. Hopefully that will reduce the chances that we'll have the same issue.

I noticed that you can't do the POST request with provider: "lb" as a request param on /currency -- not sure how flexible those should be or if either is leftovers from something..

anyways.. I think this is at least better.