Fix of bugfix to #1145

Updated list of word breaking characters for Readline completion need to omit space character ` ` as it breaks completion of `String` instance containing one or more spaces:
```ruby
irb> 'test me'.un<press TAB for completions>
me'.unbind      me'.uniq        me'.unpack      me'.untrust     
me'.ungetbyte   me'.uniq!       me'.unshift     me'.untrusted?  
me'.ungetc      me'.unlock      me'.untaint
```

See, leading string is broken by space and completion is improperly processed and unrelated methods are offered.

With the fix, completion starts to work properly again:
```ruby
irb> 'test me'.un<press TAB for completions>
'test me'.upcase   'test me'.upcase!  'test me'.upto
```

I believe this fix won't touch completions for other objects. Also see no reason to omit backslash character `\\` from the default list.

Take care