Added tests for formula evaluation

* Minor fixes as detected by the tests

t/fixtures/formula/formula.txt

@@ -0,0 +1,7 @@
+| Item     | Cost  |
+|----------+-------|
+| Bread    |    20 |
+| Tomatoes |     5 |
+| Pasta    |   100 |
+| Total    |     0 |
+/* tmf: $5,2=Sum(1:-1) */

t/fixtures/formula/sample.txt

@@ -0,0 +1,7 @@
+| Item     | Cost |
+|----------+------|
+| Bread    |   20 |
+| Tomatoes |    5 |
+| Pasta    |  100 |
+| Total    |      |
+| Test     |      |

t/spreadsheet.vim

@@ -93,4 +93,41 @@ describe 'spreadsheet'
       call tablemode#spreadsheet#DeleteColumn()
       Expect tablemode#spreadsheet#ColumnCount('.') == 1
     end
+  end
+
+  describe 'Formulas'
+    describe 'Add Formula'
+      before
+        new
+        read t/fixtures/formula/sample.txt
+      end
+
+      it 'should add a formula successfully'
+        call cursor(6, 15)
+        call tablemode#spreadsheet#AddFormula("Sum(1:4)")
+        Expect tablemode#spreadsheet#GetCell() == '125.0'
+        call cursor(8, 15)
+        Expect getline('.') == '/* tmf: $5,2=Sum(1:4) */'
+
+        call cursor(7, 15)
+        call tablemode#spreadsheet#AddFormula("Sum(1:-1)")
+        Expect tablemode#spreadsheet#GetCell() == '250.0'
+        call cursor(8, 15)
+        Expect getline('.') == '/* tmf: $5,2=Sum(1:4) ; $6,2=Sum(1:-1) */'
+      end
+    end
+
+    describe 'Evaluate Formula'
+      before
+        new
+        read t/fixtures/formula/formula.txt
+      end
+
+      it 'should evaluate the formula successfull'
+        call cursor(6, 15)
+        call tablemode#spreadsheet#EvaluateFormulaLine()
+        Expect &modified == 1
+        Expect tablemode#spreadsheet#GetCell() == '125.0'
+      end
+    end
   end