Tighten Time.new(string) parsing

	Disallow:

	* Only year-month
	* Only year-month-day
	* Preceding whitespace
	* Trailing whitespace

	Fixes [Bug #19293]
	---
	 test/ruby/test_time.rb | 13 +++++++++++++
	 time.c                 | 13 ++++++++++++-
	 2 files changed, 25 insertions(+), 1 deletion(-)