Ignore empty Accept-Language header value

Issue: SPR-15603

Author: rstoyanchev

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -467,7 +467,7 @@ public void setAcceptLanguage(List<Locale.LanguageRange> languages) {
 	 */
 	public List<Locale.LanguageRange> getAcceptLanguage() {
 		String value = getFirst(ACCEPT_LANGUAGE);
-		return value != null ? Locale.LanguageRange.parse(value) : Collections.emptyList();
+		return StringUtils.hasText(value) ? Locale.LanguageRange.parse(value) : Collections.emptyList();
 	}
 
 	/**

spring-web/src/test/java/org/springframework/http/HttpHeadersTests.java

@@ -447,6 +447,12 @@ public void acceptLanguage() {
 		assertEquals(Locale.FRANCE, headers.getAcceptLanguageAsLocales().get(0));
 	}
 
+	@Test // SPR-15603
+	public void acceptLanguageWithEmptyValue() throws Exception {
+		this.headers.set(HttpHeaders.ACCEPT_LANGUAGE, "");
+		assertEquals(Collections.emptyList(), this.headers.getAcceptLanguageAsLocales());
+	}
+
 	@Test
 	public void contentLanguage() {
 		headers.setContentLanguage(Locale.FRANCE);