[aws|ec2] describe_availability_zones parser handles nested <item> tags

This patch fixes the parser so there is no empty {'messageSet' => []}
value.

At the moment, AWS is returning a body like that is incorrectly parsed:

     <availabilityZoneInfo>
        ...
        <item>
            <zoneName>us-east-1b</zoneName>
            <zoneState>impaired</zoneState>
            <regionName>us-east-1</regionName>
            <messageSet>
                <item>
                    <message>We have restored the ability to launch new
EC2 instances in this Availability Zone. At this point, customers
should be able to launch instances in any Availability Zone in the
US-EAST-1 region. We are continuing to restore impaired volumes and
their attached instances.</message>
                </item>
            </messageSet>

lib/fog/aws/parsers/compute/describe_availability_zones.rb

@@ -4,6 +4,13 @@ module Fog
       module AWS
 
         class DescribeAvailabilityZones < Fog::Parsers::Base
+          def start_element(name, attrs = [])
+            case name
+            when 'messageSet'
+              @in_message_set = true
+            end
+            super
+          end
 
           def reset
             @availability_zone = { 'messageSet' => [] }
@@ -13,14 +20,18 @@ module Fog
           def end_element(name)
             case name
             when 'item'
-              @response['availabilityZoneInfo'] << @availability_zone
+              unless @in_message_set
-              @availability_zone = { 'messageSet' => [] }
+                @response['availabilityZoneInfo'] << @availability_zone
+                @availability_zone = { 'messageSet' => [] }
+              end
             when 'message'
               @availability_zone['messageSet'] << value
             when 'regionName', 'zoneName', 'zoneState'
               @availability_zone[name] = value
             when 'requestId'
               @response[name] = value
+            when 'messageSet'
+              @in_message_set = false
             end
           end