Some simplifications and code polish

[Philipp91]

See the individual commit messages and diffs.

Breaking changes:

• ext-json is now declared as a dependency. It was actually a dependency before, but now it's declared, which might break some builds.
• Request::getBody() was renamed to Request::buildJsonBody(). Most clients won't care, as they don't call this function.
• FcmClient::send() does not throw GuzzleException anymore, but now wraps it into FcmClientException (same exception type as when the server returns an error).