Avoid infinite loops when calling inspect

Following my earlier changes, some slightly buggy code in one of my
applications triggered this. The problem went away once the code was
corrected but it is obviously worth avoiding this to start with.

lib/her/model/introspection.rb

@@ -12,14 +12,26 @@ module Introspection
       #   @user = User.find(1)
       #   p @user # => #<User(/users/1) id=1 name="Tobias Fünke">
       def inspect
+        first = Thread.current[:her_inspect_objects].nil?
+        Thread.current[:her_inspect_objects] = [] if first
+
         resource_path = begin
           request_path
         rescue Her::Errors::PathError => e
           joined = e.missing_parameters.map { |m| "`#{m}`" }.join(", ")
           "<unknown path, missing #{joined}>"
         end
 
-        "#<#{self.class}(#{resource_path}) #{attributes.keys.map { |k| "#{k}=#{attribute_for_inspect(send(k))}" }.join(" ")}>"
+        result = "#<#{self.class}(#{resource_path}) "
+
+        if Thread.current[:her_inspect_objects].include?(self)
+          result << '...>'
+        else
+          Thread.current[:her_inspect_objects] << self
+          result << attributes.keys.map { |k| "#{k}=#{attribute_for_inspect(send(k))}" }.join(' ') + '>'
+        end
+      ensure
+        Thread.current[:her_inspect_objects] = nil if first
       end
 
       private