Feat: GetResponse에 모든 요청 시 ContentType을 application/json으로 설정 하도록 설정 추가

[#45]

src/main/java/com/careerzip/global/newsletter/GetResponseClient.java

@@ -1,17 +1,17 @@
 package com.careerzip.global.newsletter;
 
+import com.nimbusds.common.contenttype.ContentType;
 import lombok.Getter;
 import lombok.RequiredArgsConstructor;
-import org.springframework.http.HttpEntity;
-import org.springframework.http.HttpHeaders;
-import org.springframework.http.HttpMethod;
-import org.springframework.http.ResponseEntity;
+import org.springframework.http.*;
 import org.springframework.stereotype.Component;
 import org.springframework.web.client.RestTemplate;
 import org.springframework.web.util.UriComponentsBuilder;
 
 import java.net.URI;
 
+import static org.springframework.http.MediaType.APPLICATION_JSON_VALUE;
+
 @RequiredArgsConstructor
 @Getter
 @Component
@@ -30,6 +30,7 @@ public <T> T getRequest(String requestPath, Class<T> payloadType) {
     private HttpEntity createRequestEntity() {
         HttpHeaders headers = new HttpHeaders();
         headers.add(getResponseProperties.getAuthHeaderName(), getResponseProperties.getApiKey());
+        headers.add(HttpHeaders.CONTENT_TYPE, APPLICATION_JSON_VALUE);
         return new HttpEntity(headers);
     }