Lowercase the region for signing

The SigV4 signing mechanism uses a lowercase region when constructing
the string to sign. This change ensures that we always use the
lowercase region in our own construction.

Author: dchenbec

src/main/java/software/aws/mcs/auth/SigV4AuthProvider.java

@@ -156,10 +156,9 @@ public SigV4AuthProvider(@NotNull AwsCredentialsProvider credentialsProvider, fi
         if (region == null) {
             DefaultAwsRegionProviderChain chain = new DefaultAwsRegionProviderChain();
             Region defaultRegion = chain.getRegion();
-            this.signingRegion = defaultRegion.toString();
-
+            this.signingRegion = defaultRegion.toString().toLowerCase();
         } else {
-            this.signingRegion = region;
+            this.signingRegion = region.toLowerCase();
         }
 
         if (this.signingRegion == null) {