A mismatch of the strip option in the validate API

Hi, funcool. Thank you for your creating nice validation library.

I've found a mismatch between document and its work.

`validate` API's comment says,
> This function by default strips all data that are not defined in
> schema, but this behavior can be changed by passing `{:strip false}`
> as third argument.

but actually default value of strip is false.
https://github.com/funcool/struct/blob/master/src/struct/core.cljc#L129

Which have you intended?