Fix Response.InputStreamBody missing toString implementation (#1022)

Fixes #981

Author: tomdaly

core/src/main/java/feign/Response.java

@@ -17,6 +17,7 @@
 import static feign.Util.checkNotNull;
 import static feign.Util.checkState;
 import static feign.Util.decodeOrDefault;
+import static feign.Util.toByteArray;
 import static feign.Util.valuesOrEmpty;
 
 import java.io.ByteArrayInputStream;
@@ -273,6 +274,15 @@ public Reader asReader(Charset charset) throws IOException {
     public void close() throws IOException {
       inputStream.close();
     }
+
+    @Override
+    public String toString() {
+      try {
+        return new String(toByteArray(inputStream), UTF_8);
+      } catch (Exception e) {
+        return super.toString();
+      }
+    }
   }
 
   private static final class ByteArrayBody implements Response.Body {