Fix for ProjectMerger.mergeMavenPlugins Xml Declarations Issue

[jacobdanner]

Proposed Fix for ProjectMerger.mergeMavenPlugins Issue see: https://github.com/spring-projects/spring-cli/issues/217

Discovered while working on a project that was trying to merge some openapi plugin tooling. The ConversionUtil code was returning strings with xml declarations and that was breaking and not working as expectd in the openrewrite recipes that modify the pom's plugins

• Added a Test for ConversionUtil to demonstrate existing behavior
• Added Test for expected ConversionUtil output
• Updated ConversionUtil to avoid returning dependencies with default dependency content
• Updated MavenModificationTests with additional validation around merged/added plugins