String attribute's value 'yes' interpreted as date

I have the following UDA:
```
$ task show needfollowup
Config Variable Value
uda.needfollowup.default no
uda.needfollowup.label Need-Follow-Up
uda.needfollowup.type string
uda.needfollowup.values yes,no
urgency.uda.needfollowup.yes.coefficient 5
```

It's impossible to filter on its "yes" value:

```
$ task needfollowup:yes next
'no' is not a valid date in the 'Y-M-D' format.
```

**NB**: Those filters work nonetheless...:
```
$ task needfollowup:no next
$ task needfollowup.not:no next
```

```
$ task diag

task 2.5.3
Platform: Linux

Compiler
Version: 8.3.0
Caps: +stdc +stdc_hosted +LP64 +c8 +i32 +l64 +vp64 +time_t64
Compliance: C++17

Build Features
Built: Jan 25 2021 22:54:14
CMake: 3.13.4
libuuid: libuuid + uuid_unparse_lower
libgnutls: n/a
Build type: release

Configuration
File: /home/l0f4r0/.taskrc (found), 2416 bytes, mode 100660
Data: /home/l0f4r0/.task (found), dir, mode 40750
Locking: Enabled
GC: Enabled
$EDITOR: /usr/bin/vim
Server:
CA: -
Certificate: -
Key: -
Trust: strict
Ciphers: NORMAL
Creds:

Hooks
System: Enabled
Location: /home/l0f4r0/.task/hooks
(-none-)

Tests
Terminal: 323x70
Dups: Scanned 72 tasks for duplicate UUIDs:
No duplicates found
Broken ref: Scanned 72 tasks for broken references:
No broken references found
```