Parsing a URDF string injects "<urdf-string>" into file paths

## Environment
* OS Version: Ubuntu 20.04
* Source build sdf12 branch

When parsing a URDF string, parser.cc function readDoc places "" into file pathing via SetFilePath. This literal then appears in relative file paths for items such as meshes, breaking include capability.

## Description
* Expected behavior: File pathing is not arbitrarily broken via injected string data
* Actual behavior: "``" is prepended to relative paths in URDF file

It would be useful to at least have a config switch suppressing this behavior.

## Steps to reproduce

1. Launch ign gazebo
2. Process a URDF file with meshes through xacro
3. Launch robot state publisher to load the output of (2) on a topic
4. Launch ros_ign_gazebo create to spwan an entity, referencing the topic in (3)
5. Examine the resulting file paths and note "``" in mesh paths

Start in parser.cc, especially readDoc and its call to SetFilePath, then reproduce the issue using the listed URDF, xacro, and mesh-loading steps. Done means parsing a URDF string no longer causes the literal to appear in relative mesh paths, while the expected file-path behavior remains intact.

Written by the indexing model from the issue text.