Add fix for per-request ApiBase

[richardm-stripe]

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r? @pakrym-stripe

Summary

Add tests for the feature introduced in #1476 and fixes two bugs (these bugs did not get released!)

• One bug broke .toBuilderFullCopy(). It didn't copy over the new field baseUrl.
• The other bug was that new RequestOptions() defaulted to baseUrl = Stripe.getApiBase() when really it should have defaulted to null. This means that code like in Quote.java

  public InputStream pdf(Map<String, Object> params, RequestOptions options)
      throws StripeException {
    String url =
        ApiResource.fullUrl(
            Stripe.getUploadBase(),
            options,
            String.format("/v1/quotes/%s/pdf", ApiResource.urlEncodeId(this.getId())));
    return ApiResource.requestStream(ApiResource.RequestMethod.GET, url, params, options);
  }

would have regressed and been sending requests to api.stripe.com instead of files.stripe.com as intended.

[richardm-stripe]

r? @pakrym-stripe added another commit: 770bc13

It looks like I missed replacing String.format with Stripe.fullUrl in a couple non-codegenned methods.

I'm now confident I got them all. I did
grep -r Stripe.getApiBase -B1 (same for uploadBase and connectBase) to get all the places this is called. Then I did some vim magic to join any line that ended with "Stripe.fullUrl(\n" with the next line, then I deleted any line that contained "Stripe.fullUrl" then I looked at all the remaining instances of Stripe.apiBase and they were all in tests or error messages.