HttpHeaders fails to return getAllow() if it was previously set to an empty collection [SPR-11917]

[spring-projects-issues]

Oliver Drotbohm opened SPR-11917 and commented

HttpHeaders headers = new HttpHeaders();
headers.setAllow(Collections.<HttpMethod> emptySet());
headers.getAllow();

This throws an exception during the accessor call, as it can't create an HttpMethod instance from the empty String. It should rather return an empty Set instead.

java.lang.IllegalArgumentException: No enum constant org.springframework.http.HttpMethod.
  at java.lang.Enum.valueOf(Enum.java:236)
  at org.springframework.http.HttpMethod.valueOf(HttpMethod.java:27)
  at org.springframework.http.HttpHeaders.getAllow(HttpHeaders.java:229)
  …

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Affects: 3.2.9, 4.0.5

Backported to: 3.2.10