curl: Let -D merge headers in one file again

Closes #4762
Fixes #4753

src/tool_operate.c

@@ -871,7 +871,8 @@ static CURLcode single_transfer(struct GlobalConfig *global,
         if(config->headerfile) {
           /* open file for output: */
           if(strcmp(config->headerfile, "-")) {
-            FILE *newfile = fopen(config->headerfile, "wb");
+            FILE *newfile;
+            newfile = fopen(config->headerfile, per->prev == NULL?"wb":"ab");
             if(!newfile) {
               warnf(config->global, "Failed to open %s\n", config->headerfile);
               result = CURLE_WRITE_ERROR;

tests/data/test2006

@@ -86,6 +86,10 @@ Accept: */*
 Some data delivered from an HTTP resource
 </file1>
 <file2 name="log/heads2006">
+Content-Length: 496
+Accept-ranges: bytes
+
+
 HTTP/1.1 200 OK
 Date: Thu, 21 Jun 2012 14:49:01 GMT
 Server: test-server/fake

tests/data/test2007

@@ -90,6 +90,10 @@ Something delivered from an HTTP resource
 s/Last-Modified:.*//
 </stripfile2>
 <file2 name="log/heads2007">
+Content-Length: 496
+Accept-ranges: bytes
+
+
 HTTP/1.1 200 OK
 Date: Thu, 21 Jun 2012 14:50:02 GMT
 Server: test-server/fake

tests/data/test2008

@@ -82,6 +82,10 @@ Some stuff delivered from an HTTP resource
 s/Last-Modified:.*//
 </stripfile2>
 <file2 name="log/heads2008">
+Content-Length: 496
+Accept-ranges: bytes
+
+
 HTTP/1.1 200 OK
 Date: Thu, 21 Jun 2012 15:23:48 GMT
 Server: test-server/fake

tests/data/test2009

@@ -83,6 +83,10 @@ Some contents delivered from an HTTP resource
 s/Last-Modified:.*//
 </stripfile2>
 <file2 name="log/heads2009">
+Content-Length: 496
+Accept-ranges: bytes
+
+
 HTTP/1.1 200 OK
 Date: Thu, 21 Jun 2012 16:27:17 GMT
 Server: test-server/fake

tests/data/test2010

@@ -82,6 +82,10 @@ Contents delivered from an HTTP resource
 s/Last-Modified:.*//
 </stripfile2>
 <file2 name="log/heads2010">
+Content-Length: 496
+Accept-ranges: bytes
+
+
 HTTP/1.1 200 OK
 Date: Thu, 21 Jun 2012 17:37:27 GMT
 Server: test-server/fake