Use LinkedHashMap for deterministic iterations

[contextshuffling]

Test shouldPrintHeaders() asserts the string version of RequestHeader. RequestHeader class uses a HashMap to store the headers. However, HashMap does not guarantee any specific order of entries. Test can fail if the iteration order changes.
This PR proposes to fix the test by changing HashMap to LinkedHashMap that makes the iteration order deterministic.

[kortov]

I guess to change implementation for tests is not a good approach. HashMap is lighter than LinkedHashMap ofc. I guess there should be a better approach to test it though I agree on a deterministic approach

[contextshuffling]

@kortov Thanks for the feedback. I guess another approach is to assert for both orders. It gets messy when there are more but in this test, there are only two possible cases.

[kortov]

Maybe you're right with your implementation because in

feign/core/src/main/java/feign/Response.java

Line 40 in 17735a3

: new LinkedHashMap<>();

binded LinkedHashMap . And more than that, your PR is to feign.mock package, so I guess performance is not bigger issue than determenistic results.

[velo]

Change looks good, will merge as soon build passes

[contextshuffling]

@velo I fixed the format issue.