Make PlayerProfile.getProperties mutable

Most other collections returned like this is mutable, lets be consistent.

Spigot-API-Patches/0057-Basic-PlayerProfile-API.patch

@@ -1,4 +1,4 @@
-From e34afb295fa89ab296a403e9c67f1f22bfbfd03e Mon Sep 17 00:00:00 2001
+From a81aa18c534bcbca348329135c312a5725210352 Mon Sep 17 00:00:00 2001
 From: Aikar <aikar@aikar.co>
 Date: Mon, 15 Jan 2018 21:46:46 -0500
 Subject: [PATCH] Basic PlayerProfile API
@@ -7,7 +7,7 @@ Provides basic elements of a PlayerProfile to be used by future API/events
 
 diff --git a/src/main/java/com/destroystokyo/paper/profile/PlayerProfile.java b/src/main/java/com/destroystokyo/paper/profile/PlayerProfile.java
 new file mode 100644
-index 00000000..f3868f94
+index 00000000..a7b69cab
 --- /dev/null
 +++ b/src/main/java/com/destroystokyo/paper/profile/PlayerProfile.java
 @@ -0,0 +1,90 @@
@@ -37,7 +37,7 @@ index 00000000..f3868f94
 +    @Nullable UUID getId();
 +
 +    /**
-+     * @return A copy of this players properties, such as textures.
++     * @return A Mutable set of this players properties, such as textures.
 +     * Values specified here are subject to implementation details.
 +     */
 +    @Nonnull Set<ProfileProperty> getProperties();