This PR intends to fix #11638. There's a bug in the native Java generator that alters a request if the request body of an endpoint is a formatted String like XML.
By using a method specific for Strings in the request builder, I've found that it fixes the problem. The body parameter is no longer altered with surrounding double quotes and escape characters.
For further information on how to validate my work, please refer to the linked issue.

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.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

I'm tagging the technical committee responsible for the Java generator: @bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608