PBXFileReference#set_last_known_file_type() invocation has no effect for cpp and hpp files

[romanmikhailov]

I use xcodeproj gem to automatically update source files that should be listed in XCode project.

During implementation I found that PBXFileReference#set_last_known_file_type() has leaked support of the hpp and cpp files.

I checked and my version of XCode set sourcecode.cpp.cpp and sourcecode.cpp.h as lastKnownFileType attribute for hpp and cpp files respectivly.