Decode webhook request body as UTF-8 (#763)

When receiving a webhook request at bitbucket-scmsource-hook and
converting the HTTP request body to a string, always use UTF-8
even if the system default charset is different.
This is mainly relevant to a Jenkins controller running on Windows,
where the system charset might be Windows-1252, but Bitbucket Server
encodes the webhook request JSON in UTF-8.

Fix <https://github.com/jenkinsci/bitbucket-branch-source-plugin/issues/762>

Author: KalleOlaviNiemitalo

src/main/java/com/cloudbees/jenkins/plugins/bitbucket/hooks/BitbucketSCMSourcePushHookReceiver.java

@@ -28,6 +28,7 @@
 import hudson.security.csrf.CrumbExclusion;
 import hudson.util.HttpResponses;
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import javax.servlet.FilterChain;
@@ -77,7 +78,7 @@ public String getUrlName() {
      */
     public HttpResponse doNotify(StaplerRequest req) throws IOException {
         String origin = SCMEvent.originOf(req);
-        String body = IOUtils.toString(req.getInputStream());
+        String body = IOUtils.toString(req.getInputStream(), StandardCharsets.UTF_8);
         String eventKey = req.getHeader("X-Event-Key");
         if (eventKey == null) {
             return HttpResponses.error(HttpServletResponse.SC_BAD_REQUEST, "X-Event-Key HTTP header not found");