fix(ri): fix wrongly quoted options to compgen

The current implementation passes to `compgen` the option `-P ...`
through the quoted word "$prefix" where prefix='-P ...'.  However,
this is interpreted by compgen as `-P' ...'` where an extra space is
suffixed.

It was originally specified in an unquoted form `$prefix` so that `-P`
and `...` were split by the word splitting.  This was broken in commit
9ba5831 [1], which added quotes based on shellcheck SC2086.  In this
patch, we use a different approach.  We make the variable `prefix`
only contains the prefix value `...` and explicitly specify the option
`-P` at the calling site.

[1] \
9ba5831#diff-bbb83b7f6efc5ed7f97e90f27044b376b7a227f954b1ff6ff6ee0afea4bf4a36R32

completions/ri

@@ -29,7 +29,7 @@ _comp_cmd_ri__methods()
             end' | sort -u)"
     fi
     ((${#COMPREPLY[@]})) &&
-        _comp_compgen -c "$method" -- "$prefix" -W '"${COMPREPLY[@]}"'
+        _comp_compgen -c "$method" -- -P "$prefix" -W '"${COMPREPLY[@]}"'
 }
 
 # needs at least Ruby 1.8.0 in order to use -W0
@@ -85,7 +85,7 @@ _comp_cmd_ri()
         class=${cur%"$separator"*}
         method=${cur#*"$separator"}
         classes=($class)
-        prefix="-P $class$separator"
+        prefix=$class$separator
         _comp_cmd_ri__methods
         return
     fi