fix routing of microposts - important only for submitting (POST) non valid microposts

I noticed that the browser URL changes to /microposts when submitting an *empty* micropost. Even though that is no biggie per se, the user will get a routing error if he/she were to submit that URL (e.g. localhost:3000/miproposts). 
Therefore, I found it useful to change 
- the routing of micrpost post requests
- the post URL within the micropost partial.
accordingly.