Project diff returns unexpected result when project references differ.

When the difference between the projects contains project references, the `project_diff` returns unexpected results:

```ruby
project_1 = {
'projectReferences' => [
{
'ProductGroup' => {
'displayName' => 'Products',
'isa' => 'PBXGroup',
'sourceTree' => '',
'name' => 'Products',
'children' => []
},
'ProjectRef' => {
'displayName' => 'Foo.xcodeproj',
'isa' => 'PBXFileReference',
'name' => 'Foo.xcodeproj',
'path' => '../Foo/Foo.xcodeproj',
'sourceTree' => '',
'lastKnownFileType' => 'wrapper.pb-project'
}
}
]
}

project_2 = {
'projectReferences' => [
{
'ProductGroup' => {
'displayName' => 'Products',
'isa' => 'PBXGroup',
'sourceTree' => '',
'name' => 'Products',
'children' => []
},
'ProjectRef' => {
'displayName' => 'Bar.xcodeproj',
'isa' => 'PBXFileReference',
'name' => 'Bar.xcodeproj',
'path' => '../Bar/Bar.xcodeproj',
'sourceTree' => '',
'lastKnownFileType' => 'wrapper.pb-project'
}
}
]
}

diff = Differ.project_diff(project_1, project_2)

# diff:
# "projectReferences" => {
# nil => {
# "ProjectRef" => {
# "displayName" => {
# "project_1" => "Foo.xcodeproj",
# "project_2"=> "Bar.xcodeproj"
# },
# "name" => {
# "project_1" => "Foo.xcodeproj",
# "project_2" => "Bar.xcodeproj"
# },
# "path" => {
# "project_1"=> "../Foo/Foo.xcodeproj",
# "project_2"=> "../Bar/Bar.xcodeproj"
# }
# }
# }
# }
```

I narrowed down the problem to `array_diff`. The problem is that the options it receives contains `displayName` that is passed down to it from `project_diff`. But the hash elements in the array doesn't contain `displayName` in the case of project references.

It seems to me that the solution is to check in `array_diff` if the first hash element in both values contain the given key, and add this check to [this line](https://github.com/CocoaPods/Xcodeproj/blob/master/lib/xcodeproj/differ.rb#L121). This assumes that the elements are homogenous, which I'm not sure is true. Does that sound reasonable?

Start in lib/xcodeproj/differ.rb around line 121 and trace how project_diff passes options into array_diff. Reproduce the supplied projectReferences example and inspect how the missing displayName key affects matching. Done means project_diff reports the differing project references without grouping them under nil.

Written by the indexing model from the issue text.