[Feature] Adds support for specifying API version - #20

Thanks for the PR, I will review shortly.
Are there any Graph API endpoints that require this version? The documentation you linked to is referring to Azure AD Graph and not Microsoft Graph.

That is true. Is supporting Azure AD Graph within the desired scope of this project? I have a couple of other changes to support it, which complement this pull request. I might just merge all my commits into a thorough pull request instead.

It is not in scope to support Azure AD Graph; however, if there are abstractions we can introduce in the SDK to allow developers to use both, I think that would be appropriate. If you'd like to make two separate PRs, make sure you sign the CLA so that I can merge this PR.

After this merge, using the Graph API with the api_version param set, results in an error:

graph = MicrosoftGraph.new(
  api_version: '1.0',
  cached_metadata_file: File.join(MicrosoftGraph::CACHED_METADATA_DIRECTORY, 'metadata_v1.0.xml'),
  &auth_callback
)

graph.me

Traceback (most recent call last):
        1: from (irb):22
OData::ClientError (400 BadRequest: "Query parameter api-version not allowed" from "https://graph.microsoft.com/v1.0/me?api-version=1.0")