[java][resttemplate] added localVar prefix to local variable named contentType to prevent naming conflicts

[desmese]

fix #9565 and fix #9566. This bug solves the naming conflict in both #9565 and #9566. I added the localVar prefix to the local variable named contentType in the api.mustache template in the resttemplate-library. When an API specified a parameter named Content-Type this would cause a naming conflict with the variable named contentType in the api.mustasche in the resttemplate library. #9566 has more information about how to reproduce the bug.

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[wing328]

LGTM. I'll regenerate the samples and docs after merging this PR.