SqlBuilder Support Update Set (#1404)

I add update `Set` method and unit test.

e.g: 
```C#
var sb = new SqlBuilder()
       .Set("vip = @vip", new { vip })
       .Set("updatetime = @updatetime", new { updatetime })
       .Where("id = @id", new { id })
;
var template = sb.AddTemplate("update #Users /**set**/ /**where**/");
connection.Execute(template.RawSql, template.Parameters);
```

RawSql will generate :　
```
update #Users Set vip = @vip , updatetime = @updatetime\n WHERE id = @id\n
```

using Dapper Execute in sqlserver it'll reqeust by below sql command
```
exec sp_executesql N'update #Users Set vip = @vip , updatetime = @updatetime
 WHERE id = @id
',N'@vip bit,@updatetime datetime,@id int',@vip=1,@updatetime='2020-01-01 00:00:00',@id=1
```
Co-authored-by: Nick Craver <nrcraver@gmail.com>
Co-authored-by: Nick Craver <craver@stackoverflow.com>

Dapper.SqlBuilder/SqlBuilder.cs

@@ -149,5 +149,9 @@ protected SqlBuilder AddClause(string name, string sql, object parameters, strin
 
         public SqlBuilder Having(string sql, dynamic parameters = null) =>
             AddClause("having", sql, parameters, "\nAND ", "HAVING ", "\n", false);
+
+        public SqlBuilder Set(string sql, dynamic parameters = null) =>
+             AddClause("set", sql, parameters, " , ", "SET ", "\n", false);
+
     }
 }

Dapper.Tests/SqlBuilderTests.cs

-using System.Linq;
+using System;
+using System.Linq;
 using Xunit;
 
 namespace Dapper.Tests
@@ -42,5 +43,42 @@ public void TestSqlBuilderWithDapperQuery()
                 connection.Execute("drop table #Users");
             }
         }
+
+        [Fact]
+        public void TestSqlBuilderUpdateSet()
+        {
+            var id = 1;
+            var vip = true;
+            var updatetime = DateTime.Parse("2020/01/01");
+
+            var sb = new SqlBuilder()
+                   .Set("vip = @vip", new { vip })
+                   .Set("updatetime = @updatetime", new { updatetime })
+                   .Where("id = @id", new { id })
+            ;
+            var template = sb.AddTemplate("update #Users /**set**/ /**where**/");
+
+            const string createSql = @"
+                create table #Users (Id int,Name varchar(20),Age int,Country nvarchar(5),Vip bit,Updatetime datetime);
+                insert #Users (Id,Name,Age,Country) values(1,'Sam',16,'USA'),(2,'Tom',25,'UK'),(3,'Henry',14,'UK')";
+            try
+            {
+                connection.Execute(createSql);
+
+                var effectCount = connection.Execute(template.RawSql, template.Parameters);
+
+                var result = connection.QueryFirst("select * from #Users where Id = 1");
+
+                Assert.Equal("update #Users SET vip = @vip , updatetime = @updatetime\n WHERE id = @id\n", template.RawSql);
+
+
+                Assert.True((bool)result.Vip);
+                Assert.Equal(updatetime, (DateTime)result.Updatetime);
+            }
+            finally
+            {
+                connection.Execute("drop table #Users");
+            }
+        }
     }
 }