fix(client): send problem+json accept header

Author: matoous

src/main/java/com/sumup/sdk/core/ApiClient.java

@@ -132,7 +132,7 @@ private HttpRequest buildRequest(
     HttpRequest.Builder requestBuilder = HttpRequest.newBuilder();
     requestBuilder.uri(resolveUri(path, queryParams));
     requestBuilder.timeout(effectiveTimeout(requestOptions));
-    requestBuilder.header("Accept", "application/json");
+    requestBuilder.header("Accept", "application/problem+json, application/json");
     applyAuthorization(requestBuilder, requestOptions);
     applyHeaders(requestBuilder, headerParams, requestOptions);
 

src/test/java/com/sumup/sdk/core/ApiClientTest.java

@@ -93,6 +93,9 @@ void defaultRuntimeHeadersAreIncluded() {
     assertEquals("java", headers.firstValue("X-Sumup-Runtime").orElse(null));
     assertEquals(
         Runtime.version().toString(), headers.firstValue("X-Sumup-Runtime-Version").orElse(null));
+    assertEquals(
+        "application/problem+json, application/json",
+        headers.firstValue("Accept").orElse(null));
   }
 
   @Test