[Rtti] TValue doesn't handle tkAString in convertion methods
TValue.CastFromType and TValue.CastFromString doesn't handle tkAString, so simple test is failed
```
program Project1;
{$mode objfpc}
{$H+}
uses
Rtti;
var
S: string;
V, V2: TValue;
begin
S := 'str';
V := S;
if not V.TryCast(TypeInfo(UnicodeString), V2) then
begin
WriteLn('TryCast is failed');
Halt(1);
end;
end.
```
[1.patch](/uploads/c173ee086d4085b9acf3ed2dfd4d4344/1.patch)
issue