Insert serial dates

Numeric values should added as-is and not converted to strings.
This is specially important if the user is trying to add dates
in their serial form.

lib/google_drive/worksheet.rb

@@ -183,16 +183,23 @@ def [](*args)
     #   worksheet[1, 3] = "=A1+B1"
     def []=(*args)
       (row, col) = parse_cell_args(args[0...-1])
-      value = args[-1].to_s
-      validate_cell_value(value)
+      value = args[-1]
+
       reload_cells unless @cells
+      if value.is_a?(Numeric)
+        @numeric_values[[row, col]] = value
+      else
+        value = value.to_s
+        validate_cell_value(value)
+        @numeric_values[[row, col]] = nil
+      end
+
       @cells[[row, col]] = value
       @input_values[[row, col]] = value
-      @numeric_values[[row, col]] = nil
       @modified.add([row, col])
       self.max_rows = row if row > @max_rows
       self.max_cols = col if col > @max_cols
-      if value.empty?
+      if value == ''
         @num_rows = nil
         @num_cols = nil
       else
@@ -558,7 +565,7 @@ def merge_cells(top_row, left_col, num_rows, num_cols, merge_type: 'MERGE_ALL')
     # Changes the formatting of a range of cells to match the given number format.
     # For example to change A1 to a percentage with 1 decimal point:
     #   worksheet.set_number_format(1, 1, 1, 1, "##.#%")
-    # Google API reference: https://developers.google.com/sheets/api/reference/rest/v4/spreadsheets#numberformat
+    # Google API reference: https://developers.google.com/sheets/api/guides/formats
     def set_number_format(top_row, left_col, num_rows, num_cols, pattern, type: "NUMBER")
       number_format = Google::Apis::SheetsV4::NumberFormat.new(type: type, pattern: pattern)
       format = Google::Apis::SheetsV4::CellFormat.new(number_format: number_format)

test/test_google_drive.rb

@@ -44,27 +44,36 @@ def test_spreadsheet_online
     ws[1, 2] = '5'
     ws[1, 3] = '=A1+B1'
     ws[1, 4] = 13
+
+    ws.set_number_format(1, 5, 1, 2, 'yyyy-mm-dd h:mm:ss')
+    ws[1, 5] = 25569
+    ws[1, 6] = 25570.5
+
     assert { ws.max_rows == 20 }
     assert { ws.max_cols == 10 }
     assert { ws.num_rows == 1 }
-    assert { ws.num_cols == 4 }
+    assert { ws.num_cols == 6 }
     assert { ws[1, 1] == '3' }
     assert { ws[1, 2] == '5' }
-    assert { ws[1, 4] == '13' }
+    assert { ws[1, 4] == 13 }
+    assert { ws[1, 5] == 25569 }
+    assert { ws[1, 6] == 25570.5 }
     ws.save
 
     ws.reload
     assert { ws.max_rows == 20 }
     assert { ws.max_cols == 10 }
     assert { ws.num_rows == 1 }
-    assert { ws.num_cols == 4 }
+    assert { ws.num_cols == 6 }
     assert { ws[1, 1] == '3' }
     assert { ws[1, 2] == '5' }
     assert { ws[1, 3] == '8' }
     assert { ws[1, 4] == '13' }
+    assert { ws[1, 5] == '1970-01-01 0:00:00' }
+    assert { ws[1, 6] == '1970-01-02 12:00:00' }
     assert { ws[1, 1].encoding == Encoding::UTF_8 }
 
-    assert { ss.export_as_string('csv') == '3,5,8,13' }
+    assert { ss.export_as_string('csv') == '3,5,8,13,1970-01-01 0:00:00,1970-01-02 12:00:00' }
     assert { ss.available_content_types.empty? }
 
     ss2 = session.spreadsheet_by_key(ss.key)