Expose all HttpResource header values (even if commonly not multi-value)

Issue: SPR-14264

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/GzipResourceResolver.java

@@ -167,7 +167,6 @@ public class GzipResourceResolver extends AbstractResourceResolver {
 			headers.add(HttpHeaders.CONTENT_ENCODING, "gzip");
 			return headers;
 		}
-
 	}
 
 }

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/ResourceHttpRequestHandler.java

@@ -654,8 +654,19 @@ public class ResourceHttpRequestHandler extends WebContentGenerator
 		}
 		if (resource instanceof HttpResource) {
 			HttpHeaders resourceHeaders = ((HttpResource) resource).getResponseHeaders();
-			resourceHeaders.toSingleValueMap().entrySet()
+			for (Map.Entry<String, List<String>> entry : resourceHeaders.entrySet()) {
-					.stream().forEach(entry -> response.setHeader(entry.getKey(), entry.getValue()));
+				String headerName = entry.getKey();
+				boolean first = true;
+				for (String headerValue : entry.getValue()) {
+					if (first) {
+						response.setHeader(headerName, headerValue);
+					}
+					else {
+						response.addHeader(headerName, headerValue);
+					}
+					first = false;
+				}
+			}
 		}
 		response.setHeader(HttpHeaders.ACCEPT_RANGES, "bytes");
 	}

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/VersionResourceResolver.java

@@ -337,7 +337,6 @@ public class VersionResourceResolver extends AbstractResourceResolver {
 			headers.setETag("\"" + this.version + "\"");
 			return headers;
 		}
-
 	}
 
 }