Fix JSONAPI::PathSegment::Relationship#eql?

Relationship objects are used as key in @join_details in JoinManager.
This hash contains two types of objects: String and Relationship
In case of hash collisions in @join_details the method Relationship#eql? is called.
It currently fails when comparing with a string key.

Fixes #1333

Author: larskanis

lib/jsonapi/path_segment.rb

@@ -30,7 +30,7 @@ def initialize(relationship:, resource_klass: nil)
       end
 
       def eql?(other)
-        relationship == other.relationship && resource_klass == other.resource_klass
+        other.is_a?(self.class) && relationship == other.relationship && resource_klass == other.resource_klass
       end
 
       def hash
@@ -59,7 +59,7 @@ def initialize(resource_klass:, field_name:)
       end
 
       def eql?(other)
-        field_name == other.field_name && resource_klass == other.resource_klass
+        other.is_a?(self.class) && field_name == other.field_name && resource_klass == other.resource_klass
       end
 
       def delegated_field_name