Fix IEx completion crash on maps with non-atom keys

[AlexGx]

Before

iex(1)> m = %{"hello" => "world", a: 1} # any map with non-atom key
iex(2)> %{m | # press <TAB> after |<SPACE>

20:04:54.883 [error] :gen_statem #PID<0.70.0> terminating
** (ArgumentError) errors were found at the given arguments:

  * 1st argument: not an atom
...
*** ERROR: Shell process terminated! (^G to start new job) ***

After

iex(1)> m = %{"hello" => "world", a: 1, ab: 2}
iex(2)> %{m | a # <TAB> works
a:     ab:

[josevalim]

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