"exclusions.days... = off" does not work as expected

When using `timew config exclusions.days.2018_12_25 off` I expected it will disable any other exclusion for the given day, i.e. holidays and/or "work week" exclusions. But it does not do it and reports still show a day as excluded.

Here is example config:
```
~/timewarrior_test/ ❯ cat timewarrior.cfg
define holidays:
test:
2018_12_25 = test holiday

exclusions.days.2018_12_25 = off
```
And I got this in `timew week` report:

Here is another example:
```
~/timewarrior_test/ ❯ cat timewarrior.cfg
define exclusions:
tuesday = >00:00

exclusions.days.2018_12_25 = off
```

And here is the result:

```
~/timewarrior_test/ ❯ timew diag

timew 1.1.1
Platform: Darwin

Compiler:
Version: 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.10.43.1)
Caps: +stdc +stdc_hosted +LP64 +c8 +i32 +l64 +vp64 +time_t64
Compliance: C++11

Build Features
Built: Aug 24 2018 12:09:29
CMake: 3.12.1
Build type: Release

Configuration
TIMEWARRIORDB: /Users/alex/timewarrior_test/
Cfg: /Users/alex/timewarrior_test//timewarrior.cfg (-rw- 72 bytes)
Database: /Users/alex/timewarrior_test/ (drwx 160 bytes)
$EDITOR: vim
Color theme: Built-in default
00 01 02 03 04 05 06 07 08 09 10 11 12

Extensions
Location: /Users/alex/timewarrior_test//extensions (drwx 64 bytes)
(None)
```