Pluralize String with Kotlin Extension

Pluralize.kt

fun String.pluralize(count: Int): String? {
    return this.pluralize(count, null)
}

fun String.pluralize(count: Int, plural: String?): String? {
    return if (count > 1) {
        plural ?: this + 's'
    } else {
        this
    }
}

PluralizeTest.kt

import org.assertj.core.api.Assertions.assertThat
import org.junit.Test

class PluralizeTest {

    @Test
    fun pluralize() {
        assertThat("word".pluralize(0)).isEqualTo("word")
        assertThat("word".pluralize(1)).isEqualTo("word")
        assertThat("word".pluralize(2)).isEqualTo("words")
    }

    @Test
    fun pluralizeWithPlural() {
        assertThat("person".pluralize(0, "people")).isEqualTo("person")
        assertThat("person".pluralize(1, "people")).isEqualTo("person")
        assertThat("person".pluralize(2, "people")).isEqualTo("people")
    }
}