Fix admin requests with empty body to avoid response code 411

[danielimre]

Changed admin client to add empty response entity for HTTP requests without body to have proper Content-Length header set in every case. Closes #1729

[danielimre]

I haven't added any extra tests as all of these scenarios are already exercised in the acceptance tests.

[oleg-nenashev]

IMHO it is still valid

[tomakehurst]

Think we still need a test case for this. Although this code is exercised generally by the acceptance tests, there's nothing that produces the problem this is attempting to fix.

[oleg-nenashev]

@danielimre are you still interested in this pull request?

[danielimre]

@oleg-nenashev sure. Let me check if I can create a reasonable test for this.

[danielimre]

@oleg-nenashev I've put together some tests backing the changes. The simplest solution I've found without doing anything HTTP client specific stuff is to make use of wiremock itself to verify the behavior.

[oleg-nenashev]

Thanks for adding the test coverage!

[oleg-nenashev]

Thanks @danielimre!