Can't enter filenames with spaces

[David Reitter]

As it stands, it seems that one cannot save a buffer to a new file  
with a name that contains space characters, at least not in a  
straightforward way.

The reason is that the minibuffer that prompts for input does not  
accept space as an input character for the string that is read - even  
though space is a perfectly valid character is modern file systems.

minibuffer-local-completion-map binds the space character to  
minibuffer-complete-word (defined in src/keymap.c).

Frankly, complete-word might be a useful function for some, but it  
should not come at the expensive of not being able to input file  
names with normal characters in the way one would expect it.

One possible solution would be to bind minibuffer-complete-word to  
something like S-SPC by default and use a customization option to  
allow for the old binding. Ideally, however, one would preserve the  
SPC binding for all minibuffers that expect a symbol rather than a  
file name.