Update how multi-valued headers are handled

Author: sgonzalezMSFT

src/main/java/com/microsoft/aad/msal4j/HttpHelper.java

@@ -33,7 +33,7 @@ static IHttpResponse executeHttpRequest(final HttpRequest httpRequest,
                 httpResponse = httpClient.send(httpRequest);
             } catch(Exception e){
                 httpEvent.setOauthErrorCode(AuthenticationErrorCode.UNKNOWN);
-                throw new MsalException(e);
+                throw new MsalClientException(e);
             }
 
             addResponseInfoToTelemetry(httpResponse, httpEvent);

src/main/java/com/microsoft/aad/msal4j/HttpUtils.java

@@ -20,6 +20,6 @@ static String headerValue(Map<String, List<String>> headers, String headerName){
             return null;
         }
 
-        return headerValue.get(0);
+        return String.join(",", headerValue);
     }
 }

src/main/java/com/microsoft/aad/msal4j/IHttpClient.java

@@ -10,8 +10,8 @@
 public interface IHttpClient {
 
     /**
-     *  Should implement execution of outgoing http request with http client of choice. Adapts
-     *  response returned from http client to {@link IHttpResponse}
+     *  Should implement execution of outgoing HTTP request with HTTP client of choice. Adapts
+     *  response returned from HTTP client to {@link IHttpResponse}
      * @param httpRequest {@link HttpRequest}
      * @return {@link IHttpResponse}.
      * @throws Exception Non-recoverable exception. Recoverable exceptions should be handled by the