Use single instead of observable

Author: nikammerlaan

src/main/java/com/mewna/catnip/rest/handler/RestGuild.java

@@ -77,12 +77,12 @@ public RestGuild(final CatnipImpl catnip) {
 
     @Nonnull
     @CheckReturnValue
-    public Observable<Member> addGuildMember(@Nonnull final String guildId, @Nonnull final String memberId,
+    public Single<Member> addGuildMember(@Nonnull final String guildId, @Nonnull final String memberId,
                                              @Nonnull final MemberAddOptions options) {
-        return catnip().requester()
+        return Single.fromObservable(catnip().requester()
                 .queue(new OutboundRequest(Routes.ADD_GUILD_MEMBER.withMajorParam(guildId),
                         Map.of("user", memberId), options.toJson()))
-                .map(payload -> entityBuilder().createMember(guildId, payload.object()));
+                .map(payload -> entityBuilder().createMember(guildId, payload.object())));
     }
 
     @Nonnull