fixed broke ToUpdateStatement when PrefixFieldWithTableName is true

*my first ever pull request - feedback appreciated :D*

# The Bug

Setting **PrefixFieldWithTableName** to **true** messes up the update statement:

    //ExpressionVisitor
    ev.PrefixFieldWithTableName = true;
    ev.Update(a => a.Name).ToUpdateStatement(author);
    // -> UPDATE "author" SET
    
# The Test to Prove it

Added the test **Does_generate_valid_update_statement_when_fields_are_prefixed** to **ExpressionTests.cs**

    var sut = expr();
    sut.PrefixFieldWithTableName = true;

    Assert.That(sut.Update(p => p.FirstName).ToUpdateStatement(new Person {FirstName = "Guybrush"}).Trim(),
                Is.EqualTo(@"UPDATE ""Person"" SET ""Person"".""FirstName""='Guybrush'"));
                
# The Fix
When the fields, that are used to generate the statement are picked up, the prefixed fields are ignored because:

    if (updateFields.Count > 0 && !updateFields.Contains(fieldDef.Name)) continue; // added
    
**updateFields** contains the prefixed field names, but the fieldDef.Name is not prefixed.

I refactored the condition into a method called **ContainsUpdateFieldFor**

    private bool ContainsUpdateFieldFor(FieldDefinition fieldDef)
    {
        var expectedNameByFieldDefinition = PrefixFieldWithTableName
                                                    ? "{0}.{1}".Fmt(
                                                        OrmLiteConfig.DialectProvider.GetQuotedTableName(modelDef.ModelName),
                                                        fieldDef.Name)
                                                    : fieldDef.Name;

        return updateFields.Count > 0 && updateFields.Contains(expectedNameByFieldDefinition);
    }
    
Then I corrected the set fields generation with a new method called **GetUpdateAssignmentStatement**:

    var column = PrefixFieldWithTableName
                                 ? "{0}.{1}".Fmt(
                                     OrmLiteConfig.DialectProvider.GetQuotedTableName(modelDef.ModelName),
                                     OrmLiteConfig.DialectProvider.GetQuotedColumnName(fieldDef.FieldName))
                                 : OrmLiteConfig.DialectProvider.GetQuotedColumnName(fieldDef.FieldName);
    
    var assignedValue = OrmLiteConfig.DialectProvider.GetQuotedValue(value, fieldDef.FieldType);

    return return "{0}={1}".Fmt(column, assignedValue);

After skimming the other parts involved in prefixing, I guess there are more similar issues to this one...