Fixing bug where empty POST bodies would fail on NPE

Author: Paul Gray

src/main/java/org/imsglobal/lti/launch/LtiOauthSigner.java

@@ -83,7 +83,11 @@ public Map<String, String> signParameters(Map<String, String> parameters, String
     private String getRequestBody(HttpRequest req) throws IOException {
         if(req instanceof HttpEntityEnclosingRequest){
             HttpEntity body = ((HttpEntityEnclosingRequest) req).getEntity();
-            return IOUtils.toString(body.getContent());
+            if(body == null) {
+                return "";
+            } else {
+                return IOUtils.toString(body.getContent());
+            }
         } else {
             // requests with no entity have an empty string as the body
             return "";