Fixes NullPointerException when accessing a FeignException's content (#914)

Fixes NullPointerException when accessing a FeignException's content

Fixes #912 

If the content of a FeignException is null, `contentUTF8()` now returns an empty string rather than throwing a NullPointerException.

Author: robachmann

core/src/main/java/feign/FeignException.java

@@ -57,7 +57,11 @@ public byte[] content() {
   }
 
   public String contentUTF8() {
-    return new String(content, UTF_8);
+    if (content != null) {
+      return new String(content, UTF_8);
+    } else {
+      return "";
+    }
   }
 
   static FeignException errorReading(Request request, Response response, IOException cause) {

core/src/test/java/feign/FeignTest.java

@@ -526,6 +526,25 @@ public void throwsFeignExceptionIncludingBody() {
     }
   }
 
+  @Test
+  public void throwsFeignExceptionWithoutBody() {
+    server.enqueue(new MockResponse().setBody("success!"));
+
+    TestInterface api = Feign.builder()
+        .decoder((response, type) -> {
+          throw new IOException("timeout");
+        })
+        .target(TestInterface.class, "http://localhost:" + server.getPort());
+
+    try {
+      api.noContent();
+    } catch (FeignException e) {
+      assertThat(e.getMessage())
+          .isEqualTo("timeout reading POST http://localhost:" + server.getPort() + "/");
+      assertThat(e.contentUTF8()).isEqualTo("");
+    }
+  }
+
   @Test
   public void ensureRetryerClonesItself() throws Exception {
     server.enqueue(new MockResponse().setResponseCode(503).setBody("foo 1"));
@@ -882,6 +901,9 @@ void login(
     @RequestLine("POST /")
     String body(String content);
 
+    @RequestLine("POST /")
+    String noContent();
+
     @RequestLine("POST /")
     @Headers("Content-Encoding: gzip")
     void gzipBody(List<String> contents);