Allow more ISO 8601 datetime variants

This allows to use text format ISO 8601 timestampts without seconds,
with nano-, micro-, milli-seconds and Z or +/-HH:MM time zone offset.

lib/xlsxtream/row.rb

@@ -8,8 +8,10 @@ class Row
     ENCODING = Encoding.find('UTF-8')
 
     NUMBER_PATTERN = /\A-?[0-9]+(\.[0-9]+)?\z/.freeze
-    DATE_PATTERN = /\A[0-9]{4}-[0-9]{2}-[0-9]{2}\z/.freeze # yyyy-mm-dd
-    TIME_PATTERN = /\A[0-9]{4}-[0-9]{2}-[0-9]{2}T[0-9]{2}:[0-9]{2}:[0-9]{2}/.freeze # yyyy-mm-ddThh:mm:ss
+    # ISO 8601 yyyy-mm-dd
+    DATE_PATTERN = /\A[0-9]{4}-[0-9]{2}-[0-9]{2}\z/.freeze
+    # ISO 8601 yyyy-mm-ddThh:mm:ss(.s)(Z|+hh:mm|-hh:mm)
+    TIME_PATTERN = /\A[0-9]{4}-[0-9]{2}-[0-9]{2}T[0-9]{2}:[0-9]{2}(?::[0-9]{2}(?:\.[0-9]{1,9})?)?(?:Z|[+-][0-9]{2}:[0-9]{2})?\z/.freeze
 
     DATE_STYLE = 1
     TIME_STYLE = 2

test/xlsxtream/row_test.rb

@@ -74,10 +74,24 @@ def test_date_time_column
     end
 
     def test_text_date_time_column
-      row = Row.new(['1900-01-01T12:00:00+00:00'], 1, :auto_format => true)
+      candidates = [
+        '1900-01-01T12:00',
+        '1900-01-01T12:00Z',
+        '1900-01-01T12:00+00:00',
+        '1900-01-01T12:00:00+00:00',
+        '1900-01-01T12:00:00.000+00:00',
+        '1900-01-01T12:00:00.000000000Z'
+      ]
+      candidates.each do |timestamp|
+        row = Row.new([timestamp], 1, :auto_format => true)
+        actual = row.to_xml
+        expected = '<row r="1"><c r="A1" s="2"><v>2.5</v></c></row>'
+        assert_equal expected, actual
+      end
+      row = Row.new(['1900-01-01T12'], 1, :auto_format => true)
       actual = row.to_xml
       expected = '<row r="1"><c r="A1" s="2"><v>2.5</v></c></row>'
-      assert_equal expected, actual
+      refute_equal expected, actual
     end
 
     def test_time_column