Make sure dates are in UTC before sending them to PuppetDB

It seems like newer versions of PuppetDB only accept dates in UTC and
only with "Z" as time zone specifier, not "+00:00". So change to use
that instead.

Fixes #81

lib/puppetdb/astnode.rb

@@ -79,7 +79,7 @@ def evaluate(mode = [:nodes])
       value
     when :date
       require 'chronic'
-      ret = Chronic.parse(value, :guess => false).first.iso8601
+      ret = Chronic.parse(value, :guess => false).first.utc.iso8601
       fail "Failed to parse datetime: #{value}" if ret.nil?
       ret
     when :booleanop

spec/unit/puppetdb/parser_spec.rb

@@ -181,7 +181,7 @@
     end
 
     it 'should parse dates in queries' do
-      date = Time.new(2014, 9, 9).iso8601
+      date = Time.new(2014, 9, 9).utc.strftime('%FT%TZ')
       parser.parse('#node.report_timestamp<@"Sep 9, 2014"').should eq ['in', 'certname', ['extract', 'certname', ['select_nodes', ['<', 'report_timestamp', date]]]]
     end