Fix 'duplicate types' check for properties with a single type

[hanzi]

fixes #1263

The 'duplicate types' check would fail if properties with single-value TYPE parameters were present,

Background

prop.getParameter('type') would return an Array if multiple types are present, but a String if there's only one type:

TEL;TYPE="HOME,VOICE":+12345

Here, prop is going to be [ 'HOME', 'VOICE' ]. This works.

EMAIL;TYPE=HOME:john.doe@example.com

Whereas here, prop will end up as just 'HOME'. If you pass a string into new Set(), it will create an array containing all the individual characters of the string. So the check ends up effectively doing:

[ [ 'home' ] ].join('') === [ [ 'h', 'o', 'm', 'e' ] ].join('')
// returns false, because 'home' !== 'h,o,m,e'

This is fixed by moving the Array.isArray check into the filter call, so that only properties with multiple types are compared to begin with.

I removed some other checks that were unnecessary: Array.filter and Array.map always return an array, so props is already guaranteed to be one, and there's no harm in comparing empty arrays.

[codecov]

Codecov Report

Merging #1399 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1399   +/-   ##
=======================================
  Coverage   62.31%   62.31%           
=======================================
  Files           4        4           
  Lines          69       69           
=======================================
  Hits           43       43           
  Misses         26       26

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