Bugfix/13 delete zone record illegal argument exception

[ggalmazor]

FIxes #13

This PR handles a case where the client fails to delete a zone record due to being unable to parse the API's response.

After studying this unexpected behavior, we discovered that other operations leading to HTTP 204 No Content responses will also fail with the same error.

We have discarded a problem in the API responses and have established that the way we mock HTTP responses in our tests is making it difficult to reproduce the issue reported in #13.

We were able to reproduce the issue in a test by artificially modifying a response and were able to confirm that this PR is a fix for the issue.

The fix comes from completely skipping the parsing step if we get an HTTP 204, which we know it's going to be (it should be) empty.

[ggalmazor]

After discussing this issue in deep with @weppos, we've decided to either find a way to add a regression test that doesn't involve tampering with the fixtures or releasing the fix without tests.

I've updated the PR description to document what we've learned so far.

[ggalmazor]

In 80a354a I have prepared an integration test that puts in motion the full HTTP stack without requiring a live request to the API, and leverages the fixtures without tampering with them.

This won't produce false positive as the one we fixed in #32 while maintaining the run times under control.

[ggalmazor]

@weppos, I've separated the migration to end-to-end tests using intact fixtures to another PR #35

We can review and merge this one, which only contains the fix for the bug in how we deal with 204s.

[weppos]

It may be good to add an entry in the CHANGELOG
https://github.com/dnsimple/dnsimple-java/blob/master/CHANGELOG.md

[ggalmazor]

I will hold merging this one for today to give @san983 some time to review it.

[san983]

LGTM ✅