function prepareTaxonomies( taxonomyFilters ) {
	if ( ! taxonomyFilters ) {
		return {};
	}

	return objectReduce(
		taxonomyFilters,
		( result, terms, key ) => {
			// Trim whitespace and concatenate multiple terms with +
			result[ key ] = terms
				// Coerce term into a string so that trim() won't fail
				.map( term => ( term + '' ).trim().toLowerCase() )
				.join( '+' );

			return result;
		},
		{}
	);
}
Process arrays of taxonomy terms into query parameters.
All terms listed in the arrays will be required (AND behavior).

This method will not be called with any values unless we are handling
an endpoint with the filter mixin; however, since parameter handling
(and therefore `_renderQuery()`) are part of WPRequest itself, this
helper method lives here alongside the code where it is used.

@example
    prepareTaxonomies({
        tag: [ 'tag1 ', 'tag2' ], // by term slug
        cat: [ 7 ] // by term ID
    }) === {
        tag: 'tag1+tag2',
        cat: '7'
    }

@private
@param {Object} taxonomyFilters An object of taxonomy term arrays, keyed by taxonomy name
@returns {Object} An object of prepareFilters-ready query arg and query param value pairs