Fix explicitly false booleans

When a boolean option is explicitly set to `false`, e.g.:

```bash
--option=false
```

it enters slop as the string value `'false'`. This commit updates the
`BoolOption` option handler to interpret `'false'` and various other falsey
values (`false`, `'false'`, `'no'`, `'off'`, `'0'`) as logically false.

1 files changed, 7 insertions, 1 deletions

diff --git a/test/types_test.rb b/test/types_test.rb
index a45302a..ea2ba8d 100644
--- a/test/types_test.rb
+++ b/test/types_test.rb
@@ -34,9 +34,11 @@ describe Slop::BoolOption do
     @verbose  = @options.bool "--verbose"
     @quiet    = @options.bool "--quiet"
     @inversed = @options.bool "--inversed", default: true
+    @explicit = @options.bool "--explicit"
     @bloc     = @options.bool("--bloc"){|val| (@bloc_val ||= []) << val}
     @result   = @options.parse %w(--verbose --no-inversed
-                                  --bloc --no-bloc)
+                                  --bloc --no-bloc
+                                  --explicit=false)
   end
 
   it "returns true if used" do
@@ -54,6 +56,10 @@ describe Slop::BoolOption do
   it "will invert the value passed to &block via --no- prefix" do
     assert_equal [true, false], @bloc_val
   end
+
+  it "returns false when explicitly false" do
+    assert_equal false, @result[:explicit]
+  end
 end
 
 describe Slop::IntegerOption do