Fixed FirebirdSqlExpressionVisitor Trim support

Firebird doesn't support LTrim or RTrim.

for LTrim we need to use Trim(leading from str).
for RTrim we need to use Trim(trailing from str).

This avoids errors when executing code like this:

.Where<Northwind.Common.DataModel.Territory>(x =>
x.TerritoryDescription.Trim() == "Westboro");
Merge pull request #326 from madaleno/FireBird

Fixed FirebirdSqlExpressionVisitor Trim support