time.rb: fix parsing time zone in iso8601

	* lib/time.rb (Time.xmlschema): a colon in time zone designator
	  can be omitted.  [ruby-core:87277] [Bug #14790]

	time.rb: fix parsing time zone in iso8601

	* lib/time.rb (Time.xmlschema): the minute in time zone designator
	  can be omitted together with the preceding colon.
	  [ruby-core:87277] [Bug #14790]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@64431 b2dd03c8-39d4-4d8f-98ff-823fe69b080e