Summary does not consider books with multiple authors

[nus-se-bot]

Describe the bug
Books often have multiple authors, but application does not check it.

To Reproduce
Steps to reproduce the behavior:

1. Type in 'addBook n/Structure and Interpretation of Computer Programs a/Hal Abelson Jerry Sussman m/4 t/textbook'.
2. Type in 'addBook n/SICP (Part II) a/Hal Abelson m/5'. Now there should be 2 books written by Hal Abelson.
3. Type in 'summary'
4. See the result

Expected behavior
It should state that Hal Abelson is the preferred author.

Screenshots

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Reported by: @whyjayxp
Severity: Low

_{[original: nus-cs2103-AY1819S2/pe-dry-run#966]}