Fix thread safe issues

Added a local_config when using `BusinessTime::Config#with`. It copies
the current global config to current thread, apply the wanted
modifications and work as expected inside the block.
Calls to `BusinessTime::Config#config` and `BusinessTime::Config#config=`
will return the local_config if it exists.

lib/business_time/config.rb

@@ -20,13 +20,27 @@ class << self
       private
 
       def config
+        return local_config if local_config?
         Thread.main[:business_time_config] ||= default_config
       end
 
       def config=(config)
+        return self.local_config = config if local_config?
         Thread.main[:business_time_config] = config
       end
 
+      def local_config
+        Thread.current[:business_time_local_config]
+      end
+
+      def local_config=(config)
+        Thread.current[:business_time_local_config] = config
+      end
+
+      def local_config?
+        !local_config.nil?
+      end
+
       def threadsafe_cattr_accessor(name)
         define_singleton_method name do
           config[name]
@@ -131,11 +145,11 @@ def load(file)
       end
 
       def with(config)
-        old = config().dup
+        self.local_config = config().dup
         config.each { |k,v| send("#{k}=", v) } # calculations are done on setting
         yield
       ensure
-        self.config = old
+        self.local_config = nil
       end
 
       def default_config
@@ -154,7 +168,8 @@ def int_to_wday num
       end
 
       def reset
-        self.config = default_config
+        self.config       = default_config
+        self.local_config = nil
       end
 
       def deep_dup(object)

test/test_config.rb

@@ -152,5 +152,21 @@
       assert ran
       assert_equal "5:00 pm", BusinessTime::Config.end_of_workday
     end
+
+    it 'is threadsafe' do
+      old_hours                       = { fri: ['10:00', '12:00'] }
+      new_hours                       = { fri: ['10:00', '13:00'] }
+      BusinessTime::Config.work_hours = old_hours
+      t1 = Thread.new do
+        sleep 0.1
+        assert_equal BusinessTime::Config.work_hours, old_hours
+      end
+      Thread.new do
+        BusinessTime::Config.with(work_hours: new_hours) do
+          t1.join
+          assert_equal BusinessTime::Config.work_hours, new_hours
+        end
+      end.join
+    end
   end
 end