Fix test

Author: olegz

spring-cloud-function-adapters/spring-cloud-function-adapter-azure-web/src/main/java/org/springframework/cloud/function/adapter/azure/web/AzureWebProxyInvoker.java

@@ -93,7 +93,7 @@ private HttpServletRequest prepareRequest(HttpRequestMessage<Optional<String>> r
 
 
 		request.getBody().ifPresent(body -> {
-			Charset charsetEncoding = request.getHeaders().containsKey("content-encoding")
+			Charset charsetEncoding = request.getHeaders() != null && request.getHeaders().containsKey("content-encoding")
 					? Charset.forName(request.getHeaders().get("content-encoding"))
 							: StandardCharsets.UTF_8;
 			httpRequest.setContent(body.getBytes(charsetEncoding));