Append trailing space to mu4e-last-query as well

[iqbalansari]

I often start typing the search query after starting helm-mu (from mu4e header view) only to realize later that there was no space after the query picked from the header view, this change automatically adds a space to the search query irrespective of the source of initial input.

[tmalsburg]

This one was bugging me for a long time but I didn't get around fixing it.

Is it really necessary to append the space only if the query string is non-empty and doesn't end with a space? My understanding is that a double space wouldn't do any harm.

[iqbalansari]

You are right the extra spaces do not matter, but I kept it that way since it more 'stricter'. I not tied it tough and will remove the check if you think it is unnecesssary

[tmalsburg]

Well, let's keep it. It doesn't do any harm.

[tmalsburg]

Thanks for another really useful pull request!