Fix path-based GCS credentials

Providing both credential and the credential property used only the
latter, which was not expanded when credential is a file.  Also expand
credential in tests.  References #128.

src/main/java/org/gaul/s3proxy/Main.java

@@ -214,6 +214,7 @@ public final class Main {
                 credential = Files.toString(credentialFile,
                         StandardCharsets.UTF_8);
             }
+            properties.remove(Constants.PROPERTY_CREDENTIAL);
         }
 
         if (provider == null || identity == null || credential == null) {

src/test/java/org/gaul/s3proxy/TestUtils.java

@@ -16,15 +16,18 @@
 
 package org.gaul.s3proxy;
 
+import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.net.URI;
+import java.nio.charset.StandardCharsets;
 import java.util.Properties;
 import java.util.Random;
 
 import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
 import com.google.common.io.ByteSource;
+import com.google.common.io.Files;
 import com.google.common.io.Resources;
 import com.google.inject.Module;
 
@@ -147,6 +150,14 @@ final class TestUtils {
                 Constants.PROPERTY_IDENTITY);
         String credential = info.getProperties().getProperty(
                 Constants.PROPERTY_CREDENTIAL);
+        if (provider.equals("google-cloud-storage")) {
+            File credentialFile = new File(credential);
+            if (credentialFile.exists()) {
+                credential = Files.toString(credentialFile,
+                        StandardCharsets.UTF_8);
+            }
+            info.getProperties().remove(Constants.PROPERTY_CREDENTIAL);
+        }
         String endpoint = info.getProperties().getProperty(
                 Constants.PROPERTY_ENDPOINT);
         info.s3Identity = info.getProperties().getProperty(