Allow modification of uc_file headers

Changing component.

Seems reasonable. I agree that keying the array would let the alter hook can make changes more easily. We can handle the special case of the 206 Partial Content header with the key "Status", as used by drupal_add_http_header() in D7.

Give it a while, testbot is currently stalled for everything. Testbot log shows "fatal: cannot create directory at 'themes/stark': No space left on device".

Testbot first tests the branch, and if the branch has failed then individual patches will be postponed. When the branch is restarted and succeeds, then *all* patches in the queue get retested.

Currently, #5 and #7 are waiting to be tested because the branch failed due to the out of space error on the testbot server. Once that issue gets cleared up, the branch will be restarted and the patches in #0, #5 and #7 will be retested.

Not sure this header should be alterable, as the alter hook doesn't let you change whether the caller sends partial content or not:

   if ($seek_start > 0 || $seek_end < ($size - 1)) {
-    drupal_set_header('HTTP/1.1 206 Partial Content');
+    $headers['HTTP/1.1 206 Partial Content'] = NULL;
   }

...and if that's removed, the following code can be changed, which will currently fail if $value is 0:

+    $header = $name;
+    if ($value) {
+      $header .= ": $value";
+    }