HttpGraphQlTransport respects default content-type

Closes gh-359

Author: rstoyanchev

spring-graphql/src/main/java/org/springframework/graphql/client/HttpGraphQlTransport.java

@@ -24,6 +24,7 @@
 import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.graphql.GraphQlRequest;
 import org.springframework.graphql.GraphQlResponse;
+import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.util.Assert;
 import org.springframework.web.reactive.function.client.WebClient;
@@ -46,17 +47,27 @@ final class HttpGraphQlTransport implements GraphQlTransport {
 
 	private final WebClient webClient;
 
+	private final MediaType contentType;
+
 
 	HttpGraphQlTransport(WebClient webClient) {
 		Assert.notNull(webClient, "WebClient is required");
 		this.webClient = webClient;
+		this.contentType = initContentType(webClient);
+	}
+
+	private static MediaType initContentType(WebClient webClient) {
+		HttpHeaders headers = new HttpHeaders();
+		webClient.mutate().defaultHeaders(headers::putAll);
+		MediaType contentType = headers.getContentType();
+		return (contentType != null ? contentType : MediaType.APPLICATION_GRAPHQL);
 	}
 
 
 	@Override
 	public Mono<GraphQlResponse> execute(GraphQlRequest request) {
 		return this.webClient.post()
-				.contentType(MediaType.APPLICATION_GRAPHQL)
+				.contentType(this.contentType)
 				.accept(MediaType.APPLICATION_GRAPHQL, MediaType.APPLICATION_JSON)
 				.bodyValue(request.toMap())
 				.retrieve()

spring-graphql/src/test/java/org/springframework/graphql/client/WebGraphQlClientBuilderTests.java

@@ -38,6 +38,8 @@
 import org.springframework.graphql.server.webflux.GraphQlHttpHandler;
 import org.springframework.graphql.server.webflux.GraphQlWebSocketHandler;
 import org.springframework.graphql.support.DocumentSource;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.MediaType;
 import org.springframework.http.codec.ClientCodecConfigurer;
 import org.springframework.http.codec.json.Jackson2JsonDecoder;
 import org.springframework.http.server.reactive.HttpHandler;
@@ -174,6 +176,28 @@ void urlEncoding(ClientBuilderSetup builderSetup) {
 		assertThat(builderSetup.getActualRequest().getUri().toString()).isEqualTo("/graphql%20one");
 	}
 
+	@Test
+	void contentTypeDefault() {
+
+		HttpBuilderSetup setup = new HttpBuilderSetup();
+		setup.initBuilder().build().document(DOCUMENT).execute().block(TIMEOUT);
+
+		WebGraphQlRequest request = setup.getActualRequest();
+		assertThat(request.getHeaders().getContentType()).isEqualTo(MediaType.APPLICATION_GRAPHQL);
+	}
+
+	@Test
+	void contentTypeOverride() {
+
+		HttpBuilderSetup setup = new HttpBuilderSetup();
+		setup.initBuilder().header(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE).build()
+				.document(DOCUMENT).execute().block(TIMEOUT);
+
+		WebGraphQlRequest request = setup.getActualRequest();
+		assertThat(request.getHeaders().getContentType()).isEqualTo(MediaType.APPLICATION_JSON);
+
+	}
+
 	@ParameterizedTest
 	@MethodSource("argumentSource")
 	void codecConfigurerRegistersJsonPathMappingProvider(ClientBuilderSetup builderSetup) {