BUG: Doesn't properly handle `railbutton` when `instancecount` > 1

[rickhg12hs]

rocketserializer fails to recognize multiple rail buttons when instancecount is greater than one, like in:

<railbutton>
  <name>Rail Button</name>
  <id>9d8d382b-3c6d-49f0-a12e-48146cd39e2a</id>
  <preset type="RAIL_BUTTON" manufacturer="Rail-Buttons.com" partno="RB-Micro" digest="3b34718b356236806fe0fa3d75e4fcfd"/>
  <instancecount>2</instancecount>
  <instanceseparation>-0.15</instanceseparation>
  <angleoffset method="relative">180.0</angleoffset>
  <axialoffset method="bottom">-0.075</axialoffset>
  <position type="bottom">-0.075</position>
  <overridemass>5.0E-5</overridemass>
  <overridesubcomponentsmass>false</overridesubcomponentsmass>
  <finish>normal</finish>
  <material type="bulk" density="212.73036595434928">Nylon</material>
  <outerdiameter>0.004191</outerdiameter>
  <innerdiameter>0.0030353</innerdiameter>
  <height>0.0040513</height>
  <baseheight>0.0010414</baseheight>
  <flangeheight>0.0</flangeheight>
  <screwheight>0.0011811</screwheight>
</railbutton>