Cargo complains about debug_assertions optional dependencies won't work

[ishitatsuyuki]

Describe the Bug

See the descriptive Cargo warning message:

warning: Found debug_assertions in target.'cfg(...)'.dependencies. This value is not supported for selecting dependencies and will not work as expected. To learn more visit https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#platform-specific-dependencies

Steps to Reproduce

wasm-pack build

[yokra9]

I created PR( #168 ) to solve this issue. Please review to merge.