Fixed the problem with setting the column width.

Fixed the problem with setting the column width for the whole table.
Without this patch the SetColumnWidth method throws the
NullReferenceException if the column widths haven't been initialized
before. Added the test to check the issue and to see whether it's fixed.

DocX/Table.cs

@@ -274,7 +274,7 @@ namespace Novacode
             if (grid == null)
             {
                 XElement tblPr = GetOrCreate_tblPr();
-                tblPr.AddAfterSelf(XName.Get("tblGrid", DocX.w.NamespaceName));
+                tblPr.AddAfterSelf(new XElement(XName.Get("tblGrid", DocX.w.NamespaceName)));
                 grid = Xml.Element(XName.Get("tblGrid", DocX.w.NamespaceName));
             }
 

UnitTests/DocXUnitTests.cs

@@ -2731,7 +2731,22 @@ namespace UnitTests
                     Assert.AreEqual(bookmarkNames[i], result[i]);
                 }
             }
-        }        
+        }
+
+        [TestMethod]
+        public void CreateTable_WhenCalledSetColumnWidth_ReturnsExpected()
+        {
+            using (var document = DocX.Create("Set column width.docx"))
+            {
+                var table = document.InsertTable(1, 2);
+
+                table.SetColumnWidth(0, 1000);
+                table.SetColumnWidth(1, 2000);
+
+                Assert.AreEqual(1000, table.GetColumnWidth(0));
+                Assert.AreEqual(2000, table.GetColumnWidth(1));                
+            }
+        }
     }
 }