Locale.Builder.removeUnicodeLocaleAttribute(null) should throw NullPointerException

A colleague reports:

java.util.Locale.Builder.removeUnicodeLocaleAttribute(null) is documented to throw NullPointerException, but instead throws IllformedLocaleException. This patch for OpenJDK 9 fixes this:

diff -r 09b92d3067a3 src/java.base/share/classes/java/util/Locale.java
--- a/src/java.base/share/classes/java/util/Locale.java Mon Mar 13 10:24:16 2017 -0700
+++ b/src/java.base/share/classes/java/util/Locale.java Thu Mar 16 16:29:33 2017 +1100
@@ -2696,6 +2696,7 @@
          * @see #setExtension(char, String)
          */
         public Builder removeUnicodeLocaleAttribute(String attribute) {
+ Objects.requireNonNull(attribute);
             try {
                 localeBuilder.removeUnicodeLocaleAttribute(attribute);
             } catch (LocaleSyntaxException e) {

Here is a failing test that demonstrates the problem on OpenJDK8u121 (should also fail on OpenJDK 9, but this wasn't tested):

@Test
public void removeUnicodeLocaleAttribute() {
  Locale.Builder builder = new Locale.Builder();
  try {
    builder.removeUnicodeLocaleAttribute(null);
    fail();
  } catch (NullPointerException expected) {
  }
}

Test failure:

java.util.IllformedLocaleException: Ill-formed Unicode locale attribute: null [at index 0]

at java.util.Locale$Builder.removeUnicodeLocaleAttribute(Locale.java:2628)
at LocaleBuilderTest.removeUnicodeLocaleAttribute(LocaleBuilderTest.java:16)
 ...


More background:

The documentation includes:

         * @throws NullPointerException if <code>attribute</code> is null
         * @throws IllformedLocaleException if <code>attribute</code> is ill-formed

However the implementation delegates immediately to InternalLocaleBuilder. removeUnicodeLocaleAttribute() whose implementation starts as follows:
 if (attribute == null || !UnicodeLocaleExtension.isAttribute(attribute)) {

  throw new LocaleSyntaxException("Ill-formed Unicode locale attribute: " + attribute);

}

The patch applies to Locale.java rather than InternalLocaleBuilder.java because LocaleBuilder is the class that introduces the contract about NPE in its documentation.