Ensure withPreview can be called multiple times

Author: bitwiseman

src/main/java/org/kohsuke/github/GitHubClient.java

@@ -405,7 +405,7 @@ public <T> GitHubResponse<T> sendRequest(GitHubRequest request, @CheckForNull Gi
                         // Setting "Cache-Control" to "no-cache" stops the cache from supplying
                         // "If-Modified-Since" or "If-None-Match" values.
                         // This makes GitHub give us current data (not incorrectly cached data)
-                        request = request.toBuilder().withHeader("Cache-Control", "no-cache").build();
+                        request = request.toBuilder().setHeader("Cache-Control", "no-cache").build();
                         continue;
                     }
                     if (!(isRateLimitResponse(responseInfo) || isAbuseLimitResponse(responseInfo))) {

src/main/java/org/kohsuke/github/GitHubRequest.java

@@ -384,9 +384,11 @@ public B withApiUrl(String url) {
          *            the name
          * @param value
          *            the value
+         * @return the request builder
          */
-        public void setHeader(String name, String value) {
+        public B setHeader(String name, String value) {
             headers.put(name, value);
+            return (B) this;
         }
 
         /**
@@ -399,8 +401,11 @@ public void setHeader(String name, String value) {
          * @return the request builder
          */
         public B withHeader(String name, String value) {
-            setHeader(name, value);
-            return (B) this;
+            String oldValue = headers.get(name);
+            if (!StringUtils.isBlank(oldValue)) {
+                value = oldValue + ", " + value;
+            }
+            return setHeader(name, value);
         }
 
         /**