RestApiServlet: Allow HEAD requests

Change I3be4aeb009 replaced the "Get file type" REST API endpoint with
the /content endpoint. According to documentation [1], the content type
can be retrieved by making a HEAD request to the endpoint.

This doesn't work because RestApiServlet is only checking for GET, and
HEAD requests result in 404. Modify it to also allow HEAD requests.

Add a test to make sure it works.

[1] https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#get-content

Bug: Issue 4136
Change-Id: If5eaaba2fec94499f332970ce6b8c6db2ac9c571

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/HttpResponse.java

@@ -51,6 +51,16 @@ public class HttpResponse {
     return response.getStatusLine().getStatusCode();
   }
 
+  public String getContentType() {
+    return response.getFirstHeader("X-FYI-Content-Type").getValue();
+  }
+
+  public boolean hasContent() {
+    Preconditions.checkNotNull(response,
+        "Response is not initialized.");
+    return response.getEntity() != null;
+  }
+
   public String getEntityContent() throws IOException {
     Preconditions.checkNotNull(response,
         "Response is not initialized.");

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/RestSession.java

@@ -114,6 +114,9 @@ public class RestSession extends HttpSession {
     return execute(Request.Delete(url + "/a" + endPoint));
   }
 
+  public RestResponse head(String endPoint) throws IOException {
+    return execute(Request.Head(url + "/a" + endPoint));
+  }
 
   public static RawInput newRawInput(String content) {
     return newRawInput(content.getBytes(UTF_8));