Fix duplicate completion in notebook

comparaison between kernel completion and context-completin were
returning duplicate entry in some cases, due to trailing space.
sripping trailing space in comparaison prevent this.

fixes #3563

example

    import bar
    from foo im<tab>

used to propose `import` twice (actually `import` and `import[space]` as$
`import` was a token on the first line) now just complete directly to$
`import[space]`.