Module ‘Servant.Foreign’ does not export ‘header'

Running `cabal new-build` fails with:

```
Building library for servant-js-0.9.4..
[1 of 6] Compiling Servant.JS.Internal ( src/Servant/JS/Internal.hs, /home/kb/workspace/servant-js/dist-newstyle/build/x86_64-linux/ghc-8.6.4/servant-js-0.9.4/build/Servant/JS/Internal.o )
[2 of 6] Compiling Servant.JS.Axios ( src/Servant/JS/Axios.hs, /home/kb/workspace/servant-js/dist-newstyle/build/x86_64-linux/ghc-8.6.4/servant-js-0.9.4/build/Servant/JS/Axios.o )
[3 of 6] Compiling Servant.JS.Angular ( src/Servant/JS/Angular.hs, /home/kb/workspace/servant-js/dist-newstyle/build/x86_64-linux/ghc-8.6.4/servant-js-0.9.4/build/Servant/JS/Angular.o )
[4 of 6] Compiling Servant.JS.JQuery ( src/Servant/JS/JQuery.hs, /home/kb/workspace/servant-js/dist-newstyle/build/x86_64-linux/ghc-8.6.4/servant-js-0.9.4/build/Servant/JS/JQuery.o )
[5 of 6] Compiling Servant.JS.Vanilla ( src/Servant/JS/Vanilla.hs, /home/kb/workspace/servant-js/dist-newstyle/build/x86_64-linux/ghc-8.6.4/servant-js-0.9.4/build/Servant/JS/Vanilla.o )

src/Servant/JS/Vanilla.hs:12:1: warning: [-Wdodgy-imports]
Module ‘Servant.Foreign’ does not export ‘header’
|
12 | import Servant.Foreign hiding (header)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[6 of 6] Compiling Servant.JS ( src/Servant/JS.hs, /home/kb/workspace/servant-js/dist-newstyle/build/x86_64-linux/ghc-8.6.4/servant-js-0.9.4/build/Servant/JS.o )
```