title: RXJava 2 Flowable`s mergeWith not merging
body: <p>I have this method on my Room`s dao</p>
  
  <pre><code>@Query("SELECT * FROM Expense WHERE date &gt;= :initDate AND date &lt;= :endDate AND chargeableUuid = :chargeableUuid ORDER BY date")
  fun currentMonth(initDate: Long, endDate: Long, chargeableUuid: String): Flowable&lt;List&lt;Expense&gt;&gt;
  </code></pre>
  
  <p>and I am using it this way:</p>
  
  <pre><code>dao.currentMonth(firstDayOfMonth, lastDayOfMonth, account.uuid)
              .mergeWith(dao.currentMonth(firstDayOfMonth, lastDayOfMonth, card.uuid))
              .subscribe { Log.i("test", "size ${it.size}" }
  </code></pre>
  
  <p>If I execute each call separately both return one element each (different elements), but when I execute them together the log outputs that just one element is in the returned list.</p>
  
  <p>How can I merge the emitions from two Flowables into one?</p>