log about support for create_unless_exists in http protocol

Closes elastic#78.

lib/logstash/outputs/elasticsearch.rb

@@ -257,6 +257,10 @@ def register
       @protocol = LogStash::Environment.jruby? ? "node" : "http"
     end
 
+    if @protocol == "http" && @action == "create_unless_exists"
+      raise(LogStash::ConfigurationError, "action => 'create_unless_exists' is not supported under the HTTP protocol");
+    end
+
     if ["node", "transport"].include?(@protocol)
       # Node or TransportClient; requires JRuby
       raise(LogStash::PluginLoadingError, "This configuration requires JRuby. If you are not using JRuby, you must set 'protocol' to 'http'. For example: output { elasticsearch { protocol => \"http\" } }") unless LogStash::Environment.jruby?

spec/outputs/elasticsearch_spec.rb

@@ -6,11 +6,20 @@
 
 describe "outputs/elasticsearch" do
 
-  it "should register" do
-    output = LogStash::Plugin.lookup("output", "elasticsearch").new("embedded" => "false", "protocol" => "transport", "manage_template" => "false")
+  context "registration" do
 
-    # register will try to load jars and raise if it cannot find jars
-    expect {output.register}.to_not raise_error
+    it "should register" do
+      output = LogStash::Plugin.lookup("output", "elasticsearch").new("embedded" => "false", "protocol" => "transport", "manage_template" => "false")
+
+      # register will try to load jars and raise if it cannot find jars
+      expect {output.register}.to_not raise_error
+    end
+
+    it "should fail to register when protocol => http, action => create_unless_exists" do
+      output = LogStash::Plugin.lookup("output", "elasticsearch").new("protocol" => "http", "action" => "create_unless_exists")
+
+      expect {output.register}.to raise_error
+    end
   end
 
   describe "ship lots of events w/ default index_type", :elasticsearch => true do