Ignores collision tags name attribute when converting URDF files

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/64)) by adhenning (Bitbucket: [adhenning](https://bitbucket.org/%7Baa99a57e-7bf1-4a88-ad27-a3cdd36e629c%7D/)).**

The original report had attachments: [simple-robot.urdf](https://osrf-migration.github.io/sdformat-gh-pages/data/repositories/osrf/sdformat/issues/64/attachments/simple-robot.urdf)

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Converting from URDF to SDF ignores the name attribute of a link's collision tag and sets the name to _collision. This causes problems when parts of the URDF such as the contact sensor reference the collision tag by name. In the attached file, it converts leftFingerCollision to leftFinger_collision.

Conversion output:

```
#!xml

$ gzsdf print simple-robot.urdf

0 0 0 0 -0 0

0.013123 -1.7347e-18 0.03315 0 -0 0
0.049208

5.2203e-05
-4.2027e-22
1.0345e-05
6.106e-05
-2.2949e-21
1.018e-05



0 0 0 0 -0 0


1 1 1
model://GearsBot/meshes/collision/leftFinger.STL












0 0 0 0 -0 0


1 1 1
model://GearsBot/meshes/collision/leftFinger.STL



1

0
0

0

1
100

leftFingerCollision
__default_topic__

```