Bind content type text/x-h to text/x-c++hdr

*.h is commonly used for C and C++ sources.  The server's MIME type
recognizer frequently returns text/x-h for *.h files, which CM3's
clike mode does not understand. Remap to the c++hdr type just in case
its a C++ source file and a C file.

Change-Id: I37454147759bbfbcb268690dd8f00ad6bf4a8b32