dh_installman doesn't support paths with space characters

Dear Maintainer,

   * What led up to the situation?

I am practicing maint-guide chapter 5 and implemented a manpage after the
template

   * What exactly did you do (or not do) that was effective (or
     ineffective)?
I placed the manpage at "Manual Pages" directory under the source root
directory and added the following line(s) to debian/<package>.manpages:

* Manual Pages/<command>.1

   * What was the outcome of this action?

dh_installman complains of missing "Manual" file:

```
dh_installman
"Manual: No such file or directory at /usr/bin/dh_installman line 131.
debian/rules:19: recipe for target 'binary' failed
make: *** [binary] Error 2
```

   * What outcome did you expect instead?

manpage correctly installed either in the following forms in
debian/<package>.manpages:

* Manual Pages/<command>.1
* Manual\ Pages/<command>.1
* "Manual Pages/<command>.1"

Thanks!