[R] Add toString method in models

[wing328]

• added toString method to easily print out the object

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
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  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.0.1) (patch release), 6.1.x (breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
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cc @Ramanth (2019/07) @saigiridhar21 (2019/07)