GHBranch getProtection method to return correct number of reviewers #890

[jeetchoudhary]

Description

Fix GHBranch getProtection method to return correct number of reviewers configured in branch protection. Headers require change for the request.
added https://github.com/jeetchoudhary/github-api/blob/master/src/main/java/org/kohsuke/github/Previews.java#L45 to the headers

Fixes #890

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[jeetchoudhary]

@bitwiseman please review