using gsub! path, false gives an error

I cant replace a path with false. replacing with true works nicely.

```
doc.gsub! path, false
NoMethodError: undefined method `call' for nil:NilClass
from /usr/local/var/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/jsonpath-0.5.6/lib/jsonpath/enumerable.rb:95:in `yield_value'
```
