Fix parsing Withings new response format Fix OAuthSwift/OAuthSwift#663

[BJAdeGraaf]

As stated in #663, the new Withings endpoint:

1. requires a parameter action, with value "requesttoken";
2. requires parameter grant_type to equal "refresh_token" (not "authorization_code");
3. and has the response structured differently: the keys of the response are now just body and status, with the former keys, including code, all in an object down below body.

Description code change:
This PR addresses nr.3, because nr.1 and 2 don't need code modifications (see below).
The provided solution extracts the response parameters if the response dictionary contains a key body. To my knowledge, only Withings uses this particular response format.

Requirement nr.1:
To mitigate requirement nr.1, the user can append the accessTokenUrl with ?action=requesttoken, i.e.:

oauthswift = OAuth2Swift(
    consumerKey:    "********",
    consumerSecret: "********",
    authorizeUrl:   "https://account.withings.com/oauth2_user/authorize2",
    accessTokenUrl: "https://wbsapi.withings.net/v2/oauth2?action=requesttoken",
    responseType:   "code"
)

This is more of a workaround, but it works. As such, no modifications are needed in the source code. An alternative would be to introduce a boolean, specifically to accommodate Withings. My preference is to omit such special cases as much as possible. But please give feedback if this workaround is not preferred.

Requirement nr.2:
Requirement nr.2 is already implemented, and doesn't need any changes:

OAuthSwift/Sources/OAuthSwiftClient.swift

Line 201 in d85964b

parameters["grant_type"] = "refresh_token"

I have tested it, and with these changes, I can successfully authorize with Withings and can also refresh the token.