fix: Enforce Locale.US for AwsRequestSignerTest (#1111)

Date parsing can fail in other locales.

Specifically, in `en-CA` (Canada), the tests's defined date `Mon, 09 Sep 2011 23:36:00 GMT` won't
parse correctly, as the weekday short name for the locale is `Mon.` (extra period).

oauth2_http/javatests/com/google/auth/oauth2/AwsRequestSignerTest.java

@@ -41,6 +41,7 @@
 import java.net.URI;
 import java.nio.charset.StandardCharsets;
 import java.util.HashMap;
+import java.util.Locale;
 import java.util.Map;
 import org.junit.Before;
 import org.junit.Test;
@@ -64,6 +65,8 @@ public class AwsRequestSignerTest {
 
   @Before
   public void setUp() throws IOException {
+    // Required for date parsing when run in different Locales
+    Locale.setDefault(Locale.US);
     awsSecurityCredentials = retrieveAwsSecurityCredentials();
   }