config grammar: Allow spaces in [field references][like this]

Tests included.

Note about grammar compilation:
  I had to use treetop 1.4.15 (1.5.3 generates ruby code that fails to
  parse, haven't spent any time debugging)

Fixes #1513

lib/logstash/config/grammar.rb

@@ -3453,7 +3453,7 @@ module LogStashConfig
     if r1
       s2, i2 = [], index
       loop do
-        if has_terminal?('\G[^\\], ]', true, index)
+        if has_terminal?('\G[^\\],]', true, index)
           r3 = true
           @index += 1
         else

lib/logstash/config/grammar.treetop

@@ -234,7 +234,7 @@ grammar LogStashConfig
   end
 
   rule selector_element
-    "[" [^\], ]+ "]"
+    "[" [^\],]+ "]"
     <LogStash::Config::AST::SelectorElement>
   end
 

spec/conditionals/test.rb

@@ -1,6 +1,6 @@
 require "test_utils"
 
-module ConditionalFancines
+module ConditionalFanciness
   def description
     return example.metadata[:example_group][:description_args][0]
   end
@@ -23,7 +23,7 @@ end
 
 describe "conditionals" do
   extend LogStash::RSpec
-  extend ConditionalFancines
+  extend ConditionalFanciness
 
   describe "simple" do
     config <<-CONFIG
@@ -320,4 +320,24 @@ describe "conditionals" do
       end
     end
   end
+
+  describe "field references" do
+    conditional "[field with space]" do
+      sample("field with space" => "hurray") do
+        insist { subject["tags"].include?("success") }
+      end
+    end
+
+    conditional "[field with space] == 'hurray'" do
+      sample("field with space" => "hurray") do
+        insist { subject["tags"].include?("success") }
+      end
+    end
+
+    conditional "[nested field][reference with][some spaces] == 'hurray'" do
+      sample({"nested field" => { "reference with" => { "some spaces" => "hurray" } } }) do
+        insist { subject["tags"].include?("success") }
+      end
+    end
+  end
 end