Fix custom completion scripts (nushell#959)

* Fix fish completer returning empty string

* Fix completer override with fallback example

* Fix fish completer edge cases

Wgeb the first element of the completion list doesn't have a
description, `from tsv` throws an error. When using the `--flexible`
flag, the second column disappears.

This solution adds headers to the text, that way we'll get the two
columns. As a bonus, we avoid having to rename the columns :)

book/custom_completions.md

@@ -166,10 +166,9 @@ Multiple completers can be defined as such:
 ```nu
 let multiple_completers = {|spans|
  {
- $spans.0: { default_completer $spans | from json } # default
- ls: { ls_completer $spans | from json }
- git: { git_completer $spans | from json }
- } | each {|it| do $it}
+ ls: $ls_completer
+ git: $git_completer
+ } | get -i $spans.0 | default $default_completer | do $in $spans
 }
 ```
 
@@ -178,10 +177,8 @@ This example shows an external completer that uses the `fish` shell's `complete`
 ```nu
 let fish_completer = {|spans|
  fish --command $'complete "--do-complete=($spans | str join " ")"'
- | str trim
- | split row "\n"
- | each { |line| $line | split column "\t" value description }
- | flatten
+ | $"value(char tab)description(char newline)" + $in
+ | from tsv --flexible --no-infer
 }
 ```