Refactor HTTP request handling in `SproxydFileSystem`

In several methods of the `SproxydFileSystem` class, a similar pattern
of performing an HTTP request and handling the response was used. This
commit abstracts out this common pattern, which simplifies code and
should ease unit-testing.
Merge pull request #31 from scality/refactor-http-handling

Refactor HTTP request handling in `SproxydFileSystem`
Refactor unit tests