rtags-rdm-includes crash rdm start

[SimeonEhrig]

Describe the bug
I want to set the include path for the std C++ lib at start of rdm. So I added the following line to my init file (setq rtags-rdm-includes "/usr/include/c++/5.5.0"). I use the c++-mode-hook that starts the rdm server when I open a .cpp file. Without rtags-rdm-includes the rdm server is starts and I can use rtags commands. If I set the variable, I get the error message File mode specification error: (wrong-type-argument sequencep 47) in my minibuffer and rdm is not started.

To Reproduce
Steps to reproduce the behavior:

1. Add to your init file:

;; change the path depending on your system 
(setq rtags-rdm-includes "/usr/include/c++/5.5.0")
(add-hook 'c++-mode-hook 'rtags-start-process-unless-running)

2. Reload the init file or restart emacs
3. Open a .cpp file
4. Look at the minibuffer or Message buffer

Expected behavior
The rdm server should start when I open a .cpp file and the include path should be set.

Environment (please complete the following information):

• OS/Version: Ubuntu 16.04.6
• RTags(rdm,rc): 2.34
• Emacs: 26.3
• LLVM/Clang: 8.0

Additional context
At the moment, I'm using (setq rtags-process-flags "-I/usr/include/c++/5.5.0) as workaround.

[cslux]

The :type of rtags-rdm-includes is wrong. rtags-rdm-includes MUST be a list of strings not a string.

(setq rtags-rdm-include '("/usr/include/c++/5.5.0"))

[cslux]

@SimeonEhrig Fixed via ca131c1.