fix: PostPolicyV4.PostFieldsV4.Builder.addCustomMetadataField() allows to add prefixed an not prefixed custom fields (#398)

* changes to PostPolicyV4

* PostPolicyV4.addCustomMetadataField() allows to add prefixed an not prefixed custom fields

* deprecate incorrectly named method

* fix format

* addCustomMetadataField -> setCustomMetadataField

Author: dmitry-fa

google-cloud-storage/src/main/java/com/google/cloud/storage/PostPolicyV4.java

@@ -31,8 +31,8 @@
  * @see <a href="https://cloud.google.com/storage/docs/xml-api/post-object">POST Object</a>
  */
 public final class PostPolicyV4 {
-  private String url;
-  private Map<String, String> fields;
+  private final String url;
+  private final Map<String, String> fields;
 
   private PostPolicyV4(String url, Map<String, String> fields) {
     this.url = url;
@@ -58,7 +58,7 @@ public Map<String, String> getFields() {
    *     Object Form fields</a>
    */
   public static final class PostFieldsV4 {
-    private Map<String, String> fieldsMap;
+    private final Map<String, String> fieldsMap;
 
     private PostFieldsV4(Builder builder) {
       this.fieldsMap = builder.fieldsMap;
@@ -81,10 +81,11 @@ public Map<String, String> getFieldsMap() {
     }
 
     public static class Builder {
-      private Map<String, String> fieldsMap;
+      private static final String CUSTOM_FIELD_PREFIX = "x-goog-meta-";
+      private final Map<String, String> fieldsMap;
 
       private Builder() {
-        fieldsMap = new HashMap<>();
+        this.fieldsMap = new HashMap<>();
       }
 
       public PostFieldsV4 build() {
@@ -121,7 +122,13 @@ public Builder setContentType(String contentType) {
         return this;
       }
 
+      /** @deprecated use {@link #setExpires(String)} */
+      @Deprecated
       public Builder Expires(String expires) {
+        return setExpires(expires);
+      }
+
+      public Builder setExpires(String expires) {
         fieldsMap.put("expires", expires);
         return this;
       }
@@ -136,8 +143,17 @@ public Builder setSuccessActionStatus(int successActionStatus) {
         return this;
       }
 
+      /** @deprecated use {@link #setCustomMetadataField(String, String)} */
+      @Deprecated
       public Builder AddCustomMetadataField(String field, String value) {
-        fieldsMap.put("x-goog-meta-" + field, value);
+        return setCustomMetadataField(field, value);
+      }
+
+      public Builder setCustomMetadataField(String field, String value) {
+        if (!field.startsWith(CUSTOM_FIELD_PREFIX)) {
+          field = CUSTOM_FIELD_PREFIX + value;
+        }
+        fieldsMap.put(field, value);
         return this;
       }
     }
@@ -270,8 +286,8 @@ Builder addCustomCondition(ConditionV4Type type, String field, String value) {
    *     Policy document</a>
    */
   public static final class PostPolicyV4Document {
-    private String expiration;
-    private PostConditionsV4 conditions;
+    private final String expiration;
+    private final PostConditionsV4 conditions;
 
     private PostPolicyV4Document(String expiration, PostConditionsV4 conditions) {
       this.expiration = expiration;
@@ -363,9 +379,9 @@ public enum ConditionV4Type {
    *     Policy document</a>
    */
   static final class ConditionV4 {
-    ConditionV4Type type;
-    String operand1;
-    String operand2;
+    final ConditionV4Type type;
+    final String operand1;
+    final String operand2;
 
     private ConditionV4(ConditionV4Type type, String operand1, String operand2) {
       this.type = type;