Read possible Content-Type header updates when parsing Multipart

When parsing Multipart requests, the MultPartParser did read the original HTTP headers from the `ResteasyReactiveRequestContext#serverRequest()`. As a consequence, it was unable to incorporate any changes to Content-Type header done on the server-side using ContainerRequestFilter or other mechanisms.

Author: feibl

independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/core/multipart/MultiPartParserDefinition.java

@@ -71,7 +71,7 @@ public MultiPartParserDefinition(Supplier<Executor> executorSupplier, final Path
 
     @Override
     public FormDataParser create(final ResteasyReactiveRequestContext exchange, Set<String> fileFormNames) {
-        String mimeType = exchange.serverRequest().getRequestHeader(HttpHeaders.CONTENT_TYPE);
+        String mimeType = exchange.getHttpHeaders().getHeaderString(HttpHeaders.CONTENT_TYPE);
         if (mimeType != null && mimeType.startsWith(MULTIPART_FORM_DATA)) {
             String boundary = HeaderUtil.extractQuotedValueFromHeader(mimeType, "boundary");
             if (boundary == null) {