SqlString.Trim should return the same instance for not modified string (#3340)

Author: bahusoid

src/NHibernate.Test/SqlCommandTest/SqlStringFixture.cs

@@ -263,8 +263,8 @@ public void TrimAllParam()
 			Parameter p2 = Parameter.Placeholder;
 
 			SqlString sql = new SqlString(new object[] { p1, p2 });
-			sql = sql.Trim();
 
+			Assert.That(ReferenceEquals(sql, sql.Trim()), Is.True);
 			Assert.AreEqual("??", sql.ToString());
 		}
 

src/NHibernate/SqlCommand/SqlString.cs

@@ -702,6 +702,10 @@ public SqlString Trim()
 			if (_firstPartIndex < 0) return this;
 
 			GetTrimmedIndexes(out var sqlStartIndex, out var length);
+
+			if (_sqlStartIndex == sqlStartIndex && _length == length)
+				return this;
+
 			return length > 0
 				? new SqlString(this, sqlStartIndex, length)
 				: Empty;