Unexpected result of parsing xcconfig files

For example, the `OTHER_LDFLAGS` in the xcconfig file looks like below:

```
...
OTHER_LDFLAGS=$(inherited) -ObjC -filelist $(PODS_ROOT)/Pods-MyApp.xcconfig.LinkFile.list -l"IGListKit" -l"Whatever"
...
```

And after parsing xcconfig using `Xcodeproj::Config`, the items of `OTHER_LDFLAGS` are out of order.

(Maybe we shouldn't use `Set` and `sort` here:https://github.com/CocoaPods/Xcodeproj/blob/master/lib/xcodeproj/config.rb#L124)

```
XCCONFIG = Xcodeproj::Config.new("path/to/myapp.xcconfig")
other_linker_flags = XCCONFIG.other_linker_flags
simple = other_linker_flags[:simple]
puts "simple", simple
puts "OTHER_LDFLAGS", XCCONFIG.to_hash["OTHER_LDFLAGS"]
```

> simple
#
OTHER_LDFLAGS
**$(PODS_ROOT)/Pods-MyApp.xcconfig.LinkFile.list** $(inherited) -ObjC -filelist -l"IGListKit" -l"Whatever"

Expected:
> $(inherited) -ObjC -filelist **$(PODS_ROOT)/Pods-MyApp.xcconfig.LinkFile.list** $(inherited) -ObjC -filelist -l"IGListKit" -l"Whatever" -l"IGListKit" -l"Whatever"

Start at lib/xcodeproj/config.rb#L124 and reproduce the reported Xcodeproj::Config parsing with the sample OTHER_LDFLAGS value. Check how the simple flags are collected and how to_hash formats them. Done means the parsed flags and serialized value preserve the order from the xcconfig input, including linker flags.

Written by the indexing model from the issue text.