fix a dictionary finder bug for date

It worked.
```
[Foo where:@{@"intValue": @(0), @"doubleValue": @(3.14), @"booleanValue": @(NO), @"stringValue": @"bar"}];
```
But it didn't
```
[Foo where:@{@"dateValue": [NSDate date]}];
```

and an error occurred.
```
2013-10-12 01:02:52.976 demo1[54733:a0b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unable to parse the format string "dateValue == 2013-10-11 16:02:52 +0000"'
```

If we change the code
```
if ([value isKindOfClass:[NSString class]])
    [queryString appendFormat:@"%@ == '%@'", attribute, value];
```
to
```
if ([value isKindOfClass:[NSString class]] || [value isKindOfClass:[NSDate class]])
    [queryString appendFormat:@"%@ == '%@'", attribute, value];
```
Then, another error occurred.
```
2013-10-11 22:58:00.410 demo1[49246:a0b] -[__NSCFString timeIntervalSinceReferenceDate]: unrecognized selector sent to instance 0x8d5bbb0
2013-10-11 22:58:00.413 demo1[49246:a0b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString timeIntervalSinceReferenceDate]: unrecognized selector sent to instance 0x8d5bbb0'
```

So we have to pass instance of NSDate to `predictWithFormat:`.

Thanks.