Diff for adding a sheet_count method to spreadsheet gem

add_sheet_count.diff

diff --git a/lib/spreadsheet/workbook.rb b/lib/spreadsheet/workbook.rb
index 06e17af..eef4f19 100644
--- a/lib/spreadsheet/workbook.rb
+++ b/lib/spreadsheet/workbook.rb
@@ -56,6 +56,12 @@ module Spreadsheet
       add_worksheet Worksheet.new(opts)
     end
     ##
+    # Returns the count of total worksheets present.
+    # Takes no arguments. Just returns the length of @worksheets array.
+    def sheet_count
+      @worksheets.length
+    end
+    ##
     # The Font at _idx_
     def font idx
       @fonts[idx]
diff --git a/test/workbook.rb b/test/workbook.rb
index 8869e76..4d701fc 100644
--- a/test/workbook.rb
+++ b/test/workbook.rb
@@ -17,5 +17,13 @@ module Spreadsheet
     def test_writer__default_excel
       assert_instance_of Excel::Writer::Workbook, @book.writer(@io)
     end
+    def test_sheet_count
+      @worksheet1 = Excel::Worksheet.new
+      @book.add_worksheet @worksheet1
+      assert_equal 1, @book.sheet_count
+      @worksheet2 = Excel::Worksheet.new
+      @book.add_worksheet @worksheet2
+      assert_equal 2, @book.sheet_count
+    end
   end
 end